I may be wrong here, but I think the black "H" means that hub is featured in some category or subcategory. It's a good thing. Again, I could be wrong, but I think the blue "H" means it is good enough to be featured, and maybe was at one time, but has lost its heartbeat as far as pulling traffic as opposed to those black ones. Again, this is my guess. I'm not sure, but the stats on my account would support that theory.