Here's the full scoop on the blood test. I don't know if this is a reliable site or not but it explains Pacman's case on the blood test.
Sorting Out the Pacquiao Blood Test Quagmire
Never tested positive, why does he need a blood test?
He felt that last time he took one he felt weak, whether that's possible or not, it does affect him mentally which could make or break the fight.
He is also willing to take one right after the fight, so where's the problem?