Since they eat microorganisms anyway, is there any harm in feeding them drops of blood? Are human proteins even useful to a fish? What about enzymes or toxins?
To be honest I have no idea, but I am curious as to how you would acquire the blood. If you have enough to feed fish you might as well start donating it to a hospital, they need it more than your fry.
Anyway, I would try testing out the blood. Feed it to some fry and not to others, and see how it affects them.
Tell you what. If my convicts breed....wait, if? Sorry. When my convicts breed, I'll split the fry between two very similar tanks and give them both a general fry diet, but one group will get blood also. Again, this is still theoretical and I don't know how plausible this all really will be.
Knifegill has been known to be hardcore; this is just another example of this. I can try it on my Betta fry when they arrive, should not be long now. I am not in phlebotomy classes though, will have to do it "old school style" LOL
I am very interested in this idea and the answers that follow, it makes a lot of sense, at least to me. For the squeamish it is easy to get blood from your local butcher.