Not entirely sure on how alcohol effects your blood, but it does dialate your blood vessels, and assuming mass of blood does not increase, that would reduce blood pressure and reduce density due to decreased volume (thinning).
LSD I have read has vasoconstrictive effects, which would decrease volume, increasing blood pressure and density (thickening).
However this is a very oversimplified interpretation of the effects these substances have on the vascular system. Knowing what I do, I generally avoid anything with vasoconstrictive effects while on LSD in order to prevent a hypertensive crisis, but I'm not entirely sure if that is necessarily a major concern.
Don't take my information as anything conclusive, but hopefully it can help further this discussion or help you in your own research.
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Edit: Leaving my post above as an example of why you should always verify something you read online, but just did more research, apparently "thinning" in relation to blood refers to coagulation related actions of blood??
Interesting.
Thinning lessens ability to coagulate, thickening increases ability?
Hmmm... in that respect, I'm not sure LSD has any known effects in that manner. I suppose thinning/thickening of blood is a measurement of viscosity unrelated to pressure imposed density.