Ya know, I was fine with the idea of Diesel as Black Bolt just based on appearances when that was all I knew, but now that I've actually READ some Inhumans comics, I don't like it at all. As others have pointed out, Vin's strongest asset is his voice. Without it, I don't think he has the kind of presence or depth of expression needed for this role.
Black Bolt shouldn't be played by a beefcake kind of actor like Vin or Manganiello, imo. This role needs someone who excels more at silent expression - someone who more typically plays really intelligent, sensitive characters, because those are the ones who usually show more going on behind the eyes, like you can see the wheels turning when they're not saying a thing. I like the Eric Bana suggestion above. I also find Edgar Ramirez is quite good at this as well, though I doubt he's a big enough name for it. The Tom Hardys and Michael Fassbenders of the world are generally amazing at this type of acting, but they seem to be quite tied up in other things for the foreseeable future. Still, someone closer to that ilk should be what they aim for with this character, imo, rather the big, musclebound action guys.