There is only one reason to turn down a blood test day of the fight: You know you will fail it.
Manny will fail his blood test, and Mayweather knows this. Chances are, if Mayweather was caught off guard, he would fail too. But all professional athletes know how to cycle their drug usage to get around drug tests (and this would be how Pac passed all his prior tests). Mayweather probably cycled his so he would pass and Manny would fail, screwing him over for the first planned date.
The fight will still take place, but Mayweather has already landed the first blow. He basically 'outed' his opponent as someone who would fail a strict drug test. If anything, the fight will be even bigger than before, and should easily be the highest selling PPV in the history of the world
