So much is wrong in this quotation.
No inevitable preference
Mill nowhere says or implies that those who have experienced both kind of pleasure 'inevitably' or even invariably prefer the higher pleasures. What he actually says is this :
Of two pleasures, if there be one to which all or almost all who have experience of both give a decided preference, irrespective of any feeling of moral obligation to prefer it, that is the more desirable pleasure.
If only 'almost all' who have experienced both kinds of pleasure prefer [regard as more desirable] the higher, it clearly does not follow that 'people who have sampled these higher pleasures inevitably prefer them' - all people as the quote strongly implies.
Mill makes a further concesssion :
It may be objected, that many who are capable of the higher pleasures, occasionally, under the influence of temptation, postpone them to the lower. But this is quite compatible with a full appreciation of the intrinsic superiority of the higher. Men often, from infirmity of character, make their election for the nearer good, though they know it to be the less valuable; and this no less when the choice is between two bodily pleasures, than when it is between bodily and mental. They pursue sensual indulgences to the injury of health, though perfectly aware that health is the greater good. It may be further objected, that many who begin with youthful enthusiasm for everything noble, as they advance in years sink into indolence and selfishness. But I do not believe that those who undergo this very common change, voluntarily choose the lower description of pleasures in preference to the higher. I believe that before they devote themselves exclusively to the one, they have already become incapable of the other. Capacity for the nobler feelings is in most natures a very tender plant, easily killed, not only by hostile influences, but by mere want of sustenance; and in the majority of young persons it speedily dies away if the occupations to which their position in life has devoted them, and the society into which it has thrown them, are not favourable to keeping that higher capacity in exercise. Men lose their high aspirations as they lose their intellectual tastes, because they have not time or opportunity for indulging them; and they addict themselves to inferior pleasures, not because they deliberately prefer them, but because they are either the only ones to which they have access, or the only ones which they are any longer capable of enjoying. It may be questioned whether any one who has remained equally susceptible to both classes of pleasures, ever knowingly and calmly preferred the lower; though many, in all ages, have broken down in an ineffectual attempt to combine both.
This passage explicitly recognises that persons who have experienced both kinds of pleasure can go on to prefer the lower.
Bentham and Mill
To say that Mill 'betrays Bentham' is to assume that Mill was a disciple of Bentham who was unfaithful to him in Utilitarianism (1863). As far back as 1838 Mill had put a critical distance between himself and 'the master' in his essay on Bentham, a distance increased by his highly sympathetic essay on Coleridge, a figure antipathetic to Bentham, in 1840. From the late 1830s onwards, Mill was an independent thinker and discriminator : no disciple of anybody but a robust thinker with a philosophy of his own. He did not repudiate Bentham and all his works, very far from it, but in Utilitarianism he attempted to correct Bentham's 'errors' as well as to develop his own understanding of utilitarianism .
I may be taking 'betray' too literally; Blackburn's point might be that Mill's distinction introduced an inconsistency into utiltarian hedonism. Let's follow this up in the next two sections.
Quantity of pleasure
Bentham's hedonism assumes that pleasures can be measured. This is the basis of his quantitative view of pleasure and of his project for a felicific calculus. He believes that pleasures - and pains - come in units to which a numerical value, or something approximating to it, can be assigned : pleasures and pains can be given weights, and the value of different actions and their consequences measured in terms of such weights. The weight or quantity of a pleasure or pain is determined by two factors : intensity and duration. (Other factors such as certainty, fecundity, propinquity and 'purity' are also relevant in decision-making but the quantity of pleasure is a matter only of intensity and duration.)
How realistic the prospect of Benthamite calculations of units of pleasure actually is, I leave you to decide.
Quality of pleasure
One of Mill's central points is that :
It is quite compatible with the principle of utility to recognise the fact, that some kinds of pleasure are more desirable and more valuable than others. It would be absurd that while, in estimating all other things, quality is considered as well as quantity, the estimation of pleasures should be supposed to depend on quantity alone.
Mill does not reject the idea of quantity of pleasure; nor does he hold that quality of pleasure always trumps quantity. He says merely that quality is to be taken into account as well as quantity; and that a pleasure of higher quality will generally be preferred to one of lower quality by those who have experienced both. He sees himself as remedying a deficiency in how pleasure is conceived and how hedonism is to be understood.
That quality does not always trump quantity the following situation makes clear:
Suppose you wanted to derive a conclusion about the relative
quality of two wines from the choice (rather than, say, the proclamations) of experts.
The experts prefer high quality wine to low quality wine, and more of either to less,
other things being equal. It can further be assumed that in the right conditions the
experts' choices will reflect these preferences.
So if between two bottles of wines X and Y that do not differ in volume the
experts choose wine X, we have reason to believe that it is of higher quality. To
appeal to wine X's higher quality is the most natural way of explaining the choice, as
by assumption all other things are equal. However, if there is more of the low
quality wine Y the experts might also choose that - provided they believe the gain in
overall quantity to offset the lower quality of the wine. Thus if there is more of some
wine than of the other, and an expert chooses the wine available in larger quantity,
not much definite information as to the wines' relative quality is conveyed. (Christoph Schmidt-Petri, 'Mill on Quality and Quantity', The Philosophical Quarterly (1950-), Vol. 53, No. 210 (Jan., 2003), pp. 102-104 : 103.)
This situation is not one that Mill himself describes but it is fully in line with the text of Utilitarianism.
The terminology here is 'Victorian', reminiscent of Matthew Arnold's normative stress on culture as the 'best knowledge and thought of the time' which has seemed to many a model of elitism. But Mill's concept of higher pleasures can be expressed without Victorian priggishness. He is thinking, for instance, of the lifetime of undiminished pleasure that one can derive from reading, or seeing, Shakespeare's plays as against the pleasure of
drinking a bottle of fine wine every night. He is not saying that you should not drink the wine; he is simply pointing out that reading Shakespeare will produce pleasures of imagination, reflection, a sense of the expressive power of language and so forth with which a bottle of wine, drunk and forgotten, cannot compete. Nor need we appeal to Shakespeare; in our own day Mill could offer any number of films, street theatres, performance poetry events, novels, and adventure holidays, that permanently stimulate the mind long after we have seen or read them.
J.S, Mill, Utilitarianism, 1863 : http://www.gutenberg.org/files/11224/11224-h/11224-h.htm.
Christoph Schmidt-Petri, 'Mill on Quality and Quantity', The Philosophical Quarterly (1950-), Vol. 53, No. 210 (Jan., 2003), pp. 102-104 : 103.
Roger Crisp, Routledge Philosophy Guidebook to Mill on Utilitarianism (Routledge Philosophy Guidebooks). ISBN 10: 0415109787 / ISBN 13: 9780415109789
Published by Routledge, 1997 : 28-42.
Wendy Donner, 'Mill's Utiltarianism', The Cambridge Companion to Mill, ed. John Skorupski, ISBN 10: 0521422116 / ISBN 13: 9780521422116
Published by Cambridge University Press 1998-01-13, Cambridge, 1998 : 255-92.
Geoffrey Thomas, An Introduction to Ethics: Five Central Problems of Moral Judgement, ISBN 10: 0715624318 / ISBN 13: 9780715624319
Published by Duckworth, London, UK, 1993 : 67-8.
Warnock, Mary (ed), UTILITARIANISM AND ON LIBERTY Including Mill's 'Essay on Bentham' and Selections from the Writings of Jeremy Bentham and John Austin, ISBN 10: 0631233520 / ISBN 13: 9780631233527
Published by Blackwell, 2003.
Matthew Arnold, 'Culture and Anarchy' and Other Writings (Cambridge Texts in the History of Political Thought), ISBN 10: 052137796X / ISBN 13: 9780521377966
Published by Cambridge University Press, 1993.