The dead super mutant corpses littering the Statesman Hotel during the quest Reilly's Rangers will contain blood samples for the most part. If you have done the Reilly's Rangers quest prior to completing Who Dares Wins, they will not contain the samples.
Turning in samples is one of the easiest ways to earn caps or gain experience.
Unlike the ears and fingers found on corpses through perks, the super mutants can be killed by anyone/thing after the quest is obtained and still have the blood sample on their bodies.
For some reason, Fawkes does not yield a blood sample. This is probably due to the fact the game most likely considers him a companion, instead of a super mutant, despite his body model.
Xbox 360 Occasionally, Tristan's dialogue for exchanging the blood samples for caps will go normally, but neither blood samples nor caps will be transferred. This can be solved by not speeding through Tristan's dialogue during the actual transaction.
↑The Lone Wanderer: "Is there anything else I can do to help the Brotherhood?" Tristan: "I'm outfitting your Pip-Boy with the same blood sampling device we usually give our Scouts. After you kill a Mutant, grab a sample from its corpse and bring it here. I'll see you're properly compensated for your ammo expenditure. Just keep wiping out those Super Mutants and you'll be helping plenty." (Tristan's dialogue)