I find it funny that everyones jumping on Mayweathers back saying he's scared, when he's hardly giving an impossible list of demands.
He's not saying that have to fight during Ramadan, at the peak of mount everest, having been drained of 4 pints of blood each, only if its on a full moon, and only if Obama is the ring announcer. If he was scared, he'd be making incredibly outrageous demands, maybe holding the fight in the international space station.
If Mayweather suspects Manny is cheating and thinks blood tests will either stop him, or catch him, then all manny has to do is have a miniscule vial of blood taken a couple times and boom, the fights on. Its really an incredibly basic request, that wouldn't remotely effect his performance.
As for drug testing as others have said, dozens of world champions have admitted to cheating well after their career's are long gone, some have gone to jail having been found out after the fact. If blood testing isn't standard in boxing(which it damn well should be) then drugs that can be hidden in urine would be standard, hidden from blood tests wouldn't be standard, its a hundred million dollar industry at the moment, designer drugs for very specific purposes.
Considering this is Mayweathers last fight ever, and he might lose, how would you like it if you were the P4P champion, lost your final fight and when you're 60 you find out the other guy cheated, but its too late at that stage, all for the sake of a very simple blood test.
Mayweathers "role" in boxing is the hated one, its not much different to WWF, they play roles to play up the fight and the hate to the camera, Mayweathers always been the arrogant personality in these situations.
Likewise as much as this is making sure Pacman isn't cheating, its a publicity stunt to increase the "tension" between the fighters which they'll play up at a later date pre-fight.
Before a fight the antics really aren't anything more than pure WWF playacting, but really this is Mayweathers very very last outting, wouldn't you want every assurance it was a fair fight. Is a blood test really that big an ask?