There's several strong assumptions going on that might either clarify or change your thought if you bring them directly in.
What I read in what you've got there is this:
Argument 1 - Policy
- I have sufficient expertise to know the risks involved X.
- X is not risky
- X is prohibited by policy
- Therefore the policy against X is unsound
Argument 2 - Personal Action
- Policy X is unsound
- The result of action in violation of X would be beneficial to society on a small scale [it is of relative moral value and positive]
- Committing action in violation of X would require lying.
- Truth-telling is of relative moral value.
- lying is of relative negative moral value. (as contraposition of 4)
- The value of truth-telling is greater / equal / less than the value of the blood-giving [**still needs to be clarified but I think you are saying less]
It seems like if you have utter confidence in your expertise to comment on this policy, that the only questions remaining are:
(1) Are you correct in asserting your action in contravention of policy X poses no risk? [i.e., is your policy expertise sufficient to contravene existing policy?]
(2) is lying something of relative negative moral value or absolutely wrong?
(3) if it is not absolutely wrong, is its wrongness outweighed by the rightness of blood-giving?
Analogous Policy Questions
Your answer to question (1) generates a component of moral hazard that compounds the degree to which the lying might be wrong. And it makes you wholly responsible for your choice in a way that people are not when they act in accordance with flawed policies.
To flush that out, if a country sets a speed limit too low and your speeding kills someone, that's different in terms of guilt than if the country has set it too high and you're going a legal speed kills someone. In the latter cases, your driving at that speed is licensed by the government and while you can still be complicit for killing someone, it may be a fault of the policy rather than your judgment (it could still be the fault of your judgment despite the policy). In the former cases, it's all on you. Speeding transfers society's burden of responsibility onto you. You can also consider this with drunk driving.
Similarly, giving blood against the policy transfers responsibility for the consequences onto you. Now, depending on the moral theory you subscribe to, this can refer either to the realized consequences or to the potential consequences from the perspective of sound policy. In other words, on some theories, you are acting deeply immorally if you speed / drive drunk / give blood against policy even if no negative thing actually happens. On other theories, only if something bad happens have you done wrong. This is called moral hazard, and it represents a pretty substantial problem.
After that, it boils down to the lying question and whether that's sufficiently wrong to block other behaviors
n.b., I'm not competent to speak to this policy's validity. I'm just sketching out where I see philosophical issues that matter to the problem.