No, he's not an HHOFer. There are better defencemen not in the HHOF - Mark Howe, Doug Wilson, arguably Carl Brewer, J.C. Tremblay and Randy Carlyle - than Rob Blake. Blake has four all-star team selections, which would tie him for the most among defencemen not in the HHOF, along with Brewer. But I've never viewed Blake as being any better than Carlyle - a sturdy, all-round defenceman capable of contributing in several facets to a team, but a cut below HHOF worthiness.
A Conn Smythe, another Norris, or two or three more elite regular seasons - where he ranks among the top five or six defencemen in the league - would sway a lot of voters. He would have to be a key player for a Cup champion. If he were to be a No. 5 or 6 defenceman on a Cup champ, I don't think it would make a difference.
I'll also take issue with the argument that Blake was a better defenceman than Larry Murphy. Murphy was a poised, intelligent all-round defenceman who was a post-season all-star and Norris Trophy finalist on several occasions. It didn't hurt Murphy's cause that he was a key player with two organizations that won back-to-back Stanley Cups. (Pittsburgh in 1991 and 1992, Detroit in 1997 and 1998). His performance for those two Detroit teams made the difference in his HHOF bid.