Welcome dev_willis
'Bulverism' involves subsituting causes for reasons (Brian Barbour, 'Lewis and Cambridge', Modern Philology, Vol. 96, No. 4 (May, 1999), pp. 439-484: 456, n.42.)
More particularly with reference to Lewis:
... only by looking
into Undeceptions (1971) will one discover that "Bulverism," in contrast to the usual
sort of ad hominem argument, signifies the a priori blanket dismissal of some position on the grounds that its adherents have only a (generic) motive, not any reason,
for holding it (e.g., that the belief in question is correlative to social class or gender). (Robert M. Philmus, 'Reviewed Work(s): The C.S. Lewis Encyclopedia. A Complete Guide to His Life, Thought, and Writings by Colin Duriez', Utopian Studies, Vol. 14, No. 1 (2003), pp. 189-190: 190.)
Lewis seems to suggest that the scent of the rose is in some sense (unspecified) a property of the rose of which we are aware by smelling the rose. He wants to avoid a reductionist account on which there is no such property of which we can become aware, rather the interaction of our olfactory nerves with chemical substances projected by the rose cause us to have a sensation which we call and think of as the scent of the rose.
Locke on secondary qualities comes to mind. References are to Essay concerning Human Understanding (1690):
(1) A quality of x is a power of x to produce any idea in our mind.
(II, viii, 8).
(2) Primary qualities of body are those which are utterly in-
separable from it; are such as sense finds constantly in every
perceptible particle of matter, and the mind finds inseparable
from every particle. (II. viii. 9).
(3) Secondary qualities are nothing in objects themselves but
powers to produce various sensations in us by their primary
qualities. (II. viii. 10).
(3) seems pretty much in line with the causal explanation of (our experience of) the scent of the rose outlined above. It also appears to me both plausible and correct.
Lewis has, I think, used an unfortunate example in support of a thesis better supported (if one wants to support it) in other ways: the objectivity of values. He wants to resist a naturalist, reductionist account of e.g. judgements of goodness and beauty to merely emotivist responses in which the properties of goodness and beauty are not independent of us and 'out there' but are explained away in causal terms of our psychology, quite in line (mutatis mutandis) with the causal explanation of the scent of the rose.
His mistake is, I think, not to realise that even if the scent of the rose has a causal explanation of the kind outlined, its beauty may still be an objective property - one that supervenes on the causally induced olfactory experience. I do not say beauty is an objective property but I'm sure Lewis thought it was.
Where Bulverism comes in, I assume, is through Lewis's notion that since philosophical critics of objective values, who formed the main body of the (once) prevailing British empiricist tradition, could see no reason, no adequate evidential grounds, for believing in the existence of such values (by what means could we perceive them?) and could see no reason why anyone else should believe in them either, these critics supposed that there could only be a motivation to believe in the existence of objective values. A Christian such as Lewis might be motivated by his belief in God to posit objective values while he lacked evidence-based reason for believing in them.
This was not a position Lewis accepted or could accept - this could not, he was sure, be the right account of his belief in objective values. He warns us therefore against philosophers who banket-dismiss adequate evidential grounds for a belief in such values and automatically and in his view question-beggingly look only for the motivation we might have for believing in them.
Fair enough but then, what are the adequate evidential grounds? I suppose Lewis can only point out to the arguments he put forward in his books. On the proper assessment of those arguments I make no comment.
The universal blackout is pretty much the story of British empiricist ethics from Hume to Ayer, one might say - or J.L. Mackie, Inventing Right and Wrong. If you read Mackie, you would get a more specific grasp of why the blackout was imposed and just what it was. Mackie provides most of the historical references you need. (I reserve judgement, here at least, on Mackie's moral epistemology.)