The cause, Mayweather camp has been insisting for Olympic-style blood tests for Paquiao vs. Mayweather fight and has been suggesting that it is highly suspicious that Paquiao is not willing to agree, not too close to the fight anyway. Most quotable of course was Oscar Dela Hoya's comment that Manny's punches felt very much like Mosley's and Vargas' punches, two boxers who have been linked to performance enhancing drugs.
There is a heated debate on the net between both camps but what is clear is that there is some science to be learned. In fact, when I first read the news reports about the controversy I too felt that Manny could be hiding something. But after reading further, there is in fact a big argument to the contrary.
There are actually experts who have written that blood tests are indirect tests only and would still require confirmatory urine testing. In fact, the article stated that the only use of blood tests is that they are cheaper and are good as preliminary test.
Read the "EPO Drug Testing Q&A" here.