More info that was promised- What is likely happening is that the sharks can smell the blood but it's just not in a quantity that is interesting to them especially since they don't love human blood that much (btw, I spoke with a marine biologist and mammal blood is all the same from chemical marker stand point so cow blood is a suitable substitute for human blood).
If I did the experiment again, I would want to test human blood vs.
Fishblood.