The biggest problem is in Anton Yakovlev as the extremely over the top mercenary. His facial expressions and dialogue are just so comedic that it takes the wind right out of the sails of an otherwise pretty straight forward film. Had Black plaid more to the comedic side, his character would have fit in nicely but Black is not at all a comedy. In fact, most of the moments that might get a laugh are clearly not intended that way and the very few times the film does go for a joke, it could also be taken somewhat seriously.
I like Black because it manages to accomplish what it set out to do really well. Not your big budget Hollywood film, it brings action to the forfront and hammers it home despite a laughable main antagonist.