"She is commenting on his blog."
Translation:Hun kommenterer bloggen hans.
kommenterer bloggen hans has a duality. It can both mean that she leaves a comment on his blog and that she makes a remark about his blog (on another blog or mentioning to a friend etc), but mostly the last one. kommentere på bloggen is the first alternative, she makes a comment within his blogosphere.
Thanks for your help :) P.S.: i've supposed that the preposition deals with location and not with the verb, but I've also guessed that "å kommentere" could hold a direct object as well, which in this case should have been "bloggen hans", without any preposition, and which is however translated in english as a location complement. At the end I think this is just a peculiar case related to an english way to consider the blog as a place where is possible to leave comments, rather than a possible object of commenting...