Actually, he was People that think him not realizing Walt was Heisenberg earlier is any serious knock against his overall intelligence are the not-so-bright ones.

Hank really isn't one of the brightest people on the show which is ironic considering he's a DEA Agent.

Not to mention the fact that Hank knew Walt and Jesse had a history.

Yeah exactly. Hank had a huge blind spot for Walt for those reasons.
And he never would have thought Walt would have it in him to be capable of being a meth dealing drug kingpin. He looked at Walt as just a brainy nerd type. I also think I had a lot to do with the fact that Walt definitely wouldn't have fit hanks idea of a criminal after dealing with them day after day. Walt wouldn't have matched the profile of the typical criminals hank was used to.
Its funny though, upon rewatching the show it seems so blatantly obvious that hank should have realized it sooner. The truth had to practically slap hank in the face before he put it together.
Maybe hank thinks more clearly when hes dropping a deuce lol

There was this huge mental block and mental barrier in Hank's mind that prevented him from ever even considering Walt as a suspect on account of him being family and having a very milquetoast personality.