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 a 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. fish blood.
This is the most definitive study done to date in a control lab environment on what is least and most interesting to them: The Biology of Sharks and Rays Peter A. Klimley and Steven Oerding; ISBN-13: 9780226442495. Particularly pages 134-140. Figure 6.7; Table 6.2
Summary: I've always wanted to test if sharks can really smell a single drop of blood in the water from 1 mile away. So I went to the Bahamas in shark infested waters and set up an experiment to get to the bottom of things.