Blake lacks confidence in himself. Even if he was up 6-0 5-0 40-0, I believe he'd think to himself "holy crap, i'm about to break this two-time grand slam winning kid's almost 80-something streak on clay", then proceed choke the match away with wildly low percentage shots that he had been making up to that point. Clay court matches are a not only a test of one's physical ability, but also of one's mental ability. Having one but not the other won't suffice, especially against someone like Nadal whose immense confidence in himself and his game alone has willed him to win many matches along the way of this streak.