I come close to passing out every time the doctors take blood for tests - I can't imagine having the stones to actually give blood in that manner myself. Although I wonder what they do with the blood afterward?
The lancets for testing your blood are tiny and short. They produce at most a drop or two - just enough to cover the receptive dot on the test strip. (diabetic here).
If I am going to do this I usually use the blood to anoint a candle or to underscore something written on paper. Occasionally it will go into something I drink or pour out. That small amount is enough, in my opinion, to convey the energies I am trying to work with and disposal isn't really an issue.
Mind, I don't worship any gods for whom blood sacrifice is a major deal - I mostly use it to tie magic down rather than as an element in worship.
Absent