Swedish researchers asked that exact same question. Odorants, molecules that dissolve in air and find their way into our nasal mucus membranes to interact with our olfactory receptors, must be volatile. So, first, they extracted volatile compounds from mammalian blood. Then they separated and analyzed them for their chemical composition using gas chromatography-mass spectrometry.
(Guess Chem class is good for something!)
Next, to determine which chemical is responsible for “blood smell” they tested each extract with human subjects. Turns out just one compound smells like blood: trans-4, 5-epoxy-(E)-2-decenal. Say that five times fast!
Finally, to make sure this compound could attract large, dangerous predators (to answer questions about animal behavior and olfaction, not to weaponize it…I hope), they compared the responses of wild dogs and tigers to blood, blood odorant, a fruity odorant, and a control solvent by soaking wooden blocks in the substances. Turns out dogs and tigers love blood odorant as much as the real thing.
Tigers don’t know it’s not blood!