It was revealed that Odin actually placed Thor's consciousness into the real Don Blake's body, then whisked Blake away the moment he first transformed back into Thor. Thor subconsciously created new Don Blake bodies for himself whenever he changed back after that point. Personally, I prefer the older explanation that Odin modeled a human body on Keith Kincaid, magically created the identity of Donald Blake for it, and magically changed Thor into that new identity. I don't like the idea of Blake as a separate human being. But, alas, it's canon. Not that anyone at Marvel remembers, I'm sure.
It still doesn't really make sense for him to be in a Don Blake body again, though. The only reason I can think of for that would be Thor subconsciously used the Odinpower to change his body into one that resembled Don Blake's. Maybe because it represented a simpler time for him or something. But that'd just leave him as Thor in Don Blake's body. The Odinpower would still be with him because without Asgard, it doesn't have any place else to go.