Seems like in blog_addons/blog_addons.module line 319 the query should be $query = "SELECT count(n.nid) AS mycount, n.nid, n.created FROM {node} ... GROUP BY n.nid ...

Then the next line, $postpermonth = db_result(db_query($query));
would set $postpermonth equal to mycount, which is what would appear in the block under posts for each month.

Otherwise, it gives a warning when a blog post is displayed.