As dominant as Federer was from 2004-07, I think its a major stretch to think any "what if" scenario would have resulted in him losing a slam.
The one non-clay he didn't win in that span was 2005 Oz where he lost one of the greatest matches in tennis history to Marat Safin - 7-5, 4-6, 7-5, 6-7(6), 7-9. We all know Safin was capable of some sublime tennis when at his best.
Is it possible Blake could have done it? Sure. He's always been the type of guy who goes for his shots, and when he's one, he looks unstoppable. Just watch the first two sets of the 2005 QF vs. Agassi. But, Agassi managed to find his way into the match and eventually pulled it out. Translate that to Federer, who by the 2005 US Open was a 5-time slam winner, 3-time Wimbledon champion and defending US Open champion. He was supremely confident.
Blake would not have rattled him, IMO, even with a fast start like he had in the Agassi match, nor do I think Blake could have sustained the necessary level to win 3 of 5 sets against Federer in a slam final. I don't think Blake was as talented as Safin, or capable of reaching the level of Safin's best, and it took something special for him to win in "overtime" in Australia that year.
JMO
Anything can happen, but it would have took something really special from Blake to pull it off. The chances of it happening would have been miniscule.
The most likely result would have been a straight set Federer win, or at best, a 4 setter.