Don Blake's a real person. Odin stuck Thor in his body until the moment Blake smacked the cane against a rock in that cave way back in Journey Into Mystery #83. Then Odin took Blake's body while Thor's essence recreated his own body. Ever since that point, a new Blake shell was magically created for Thor every time he transformed back. The real Blake was in suspended animation the whole time in a cave near Mount Wundagore, where Odin had left him. Because Odin, when you get right down to it, really is a jerk.
The whole thing was revealed in Thor #479, which, as far as I know, is the most current explanation of the whole Blake/Thor thing. It was explained before that that Odin had created Blake based on Keith Kincaid, but he reveals in #479 that that was a lie, and that Kincaid was just his second choice if, for some reason, Blake didn't work out. Also, trapping Thor in Blake's body wasn't just for a humility lesson, it was because Volla the prophetess had a vision that Thor would die if he remained in Asgard.
Personally, I always preferred the explanation that Blake was really just a body Odin had created for Thor, but it's in canon that Blake is a real guy. I doubt that the Blake we saw in this last issue of FF was really Don Blake, though. He seemed pretty big, and I couldn't find a cane in any of the panels. The magical bodies created for Thor after the real Blake was taken away supposedly grew in stature because Thor was subconsciously making them more like his real body, which would explain his size in FF.
But, in reality, I'm probably just giving JMS way, way too much credit, believing he might actually know all this.