Don't remember about 7, but I am sure it has to be similar to 8. You have to edit the xml file for the unigrid that shows comments. Go to CMS\CMSModules\Blogs\Tools\Comments_List.xml. In the xml add new column:
<column source="CommentPostDocumentID" caption="CommentPostDocumentID" wrap="false"></column>
Restart the CMS. Check if you can see this new column with DocumentID.