I assume you created a sub-site as a Blog called News?! Well that is what I have done anyway. You need to go to that page and then into the permission setting and assign CONTRIBUTE right.
For the info: the contribution right will allow the user to
- add comments
- manage categories
- manage comments
- manage posts
This is way too much of permissions!
Therefore, go to Manage posts (as admin) and in there go to manage permissions > set unique permission and remove the user group with the contribute permission :)
This removes the BLOG TOOL from the right side and the users can comment at same time. However, user cannot create new posts!
PS: Might bee too old this topic, but might be useful to someone