It is supposed to be "hun kommenter PÅ bloggen hans" if the solution includes "on his blog" not just "his blog". I am a native Norwegian and I know this stuff and this is really annoying, loosing Points in a Language i have spoken since 1 years old and in English which i have spoken since 5 years old..
The variation with 'på' is accepted.
It is indeed an unusual sentence, but it is possible to comment on the properties and traits of a blog, and not just 'comment' by posting a something on it.
so if it had på would that mean you are commenting on the traits of the blog or posting a comment on it?
'kommenterer på' would likely mean that you were posting a comment on it.
Since posting a message on a blog is more usual than commenting about a blog, would it not be a good idea to change the exercise by putting a på into the Norwegian?
Well... the other way round: Why is "She commented his blog" wrong? Isn't it exactly the same in english as in norwegian?
My guess is because you've got the wrong tense. I think that kommenterer is present tense.