Blount is old, he doesn't look 250 to me, and his game is right around the basket--he seems like the prototypical Eastern Conference center, who makes his living shoving small guys around and doesn't have anything to fall back on if he's facing someone who's bigger and better than him.
I like Marc Jackson a lot, but his career path doesn't suggest that he's worth the MLE, and Jackson's career is an awfully good match for Blount's to this point. I think Jackson is thicker and more of a banger, though--I'd prefer the Suns to trade a second-round pick for Jackson's $4m contract instead of signing Blount for the MLE.
Blount had a pretty good playoffs statistically, but Kenyon Martin was a monster in the same series, and the Celtics got absolutely toasted.