The blog RSS feeds do not include tags for each post. You can get an RSS feed that filters by tag, but you can't have a feed which includes all posts, and get information about the tags in those posts.

Some RSS feeds include tags using the category tag. See this blog post:

http://www.niallkennedy.com/blog/2005/02/tagaware-rss-fe.html

I have done a patch which adds support for this feature in Moodle blog RSS feeds and rsslib.php. (Not coincidentally, this also allows oublog's implementation of the feature to work in RSS feeds as well as Atom - see CONTRIB-1307.)

Since this is hopefully for 1.9, and I haven't looked at blog code much, I'd appreciate review/approval. The patch is very short.