When people talk about the health benefits of Blood donation, it is similar to Venesection (puncturing of veins) which was called FASD in old times. You can read the article Cauterization.
The difference between Hijamah & FASD is the blood itself, FASD (Blood Donation) removes fresh blood from the veins; while Hijamah removes the septic blood from underneath the skin.
FASD is helpful for diseases that are cured by extracting out the fresh blood, & those ailments are very few, but Septic blood which is the root cause of many diseases is only drained out by Hijamah. Ibn Al Qayyam wrote that “Cupping extracts the septic blood more efficiently than puncturing the veins”. A surprising fact is that the putrefied Hijamah blood can’t be donated.