T. H. Park
All their matches have been on hard courts, Blake's best surface by far and Nadal's worst by far. If they played 5 times on hard courts, 5 times on clay, and 2 times on grass, the head to head would be 9-3 in Nadal's favor. Of course Blake was never good enough on grass or clay to get far enough to play Nadal.
Even with the matches on hard courts the edge is a thing of the past. Nadal won their last 2 meetings, both on hard courts, and is likely to continue to win any future meetings between the two even on that surface. He has improved greatly since those first 3 meetings and Blake has already peaked, and if anything is going to regress with age now.
Please spare me the "if" this "if" that argument, especially for a player like Nadal. "If" there would have been no Roland Garros, Nadal would have been a one slam wonder unless he wins something outside the red stuff in the future (a good possibility). Please try to not use that argument as it will really make a great player like Nadal look pretty bad. Beating Nadal on clay is nearly impossible - he is without much exaggeration the best clay courter ever.
I do, however, agree somewhat with your 2nd paragraph as that makes more sense. Nevertheless, nothing is for certain. Nadal's knees do not look too good and his style of playing is (according to the likes of Sampras and Becker) going to have its toll on his body as it already has. So, in short I would not put too much trust in the "edge" is in the past concept for Blake. I believe that Nadal's peak was in 2008 and he will have a very tough time to top his performance of this year for the future and his body will regress faster and far more rapidly than players like Blake or Federer who have to work less to win matches (in terms of style of play).