I just finished setting up a Blue Obelisk section for Chemical blogspace, as future replacement for the current Planet Blue Obelisk (unless someone wants to take over that webpage). The only thing really missing is a RSS feed for recent posts for just the Blue Obelisk member blogs (BTW, just email me if you want to be listed as BO member with your blog too; the BO community is very open!).
For now, you will have to do with this page:
An additional flaw is that it also shows molecules for other blogs.
Update: the RSS feed for a specific category was already available, but just not from the FireFox URL bar. Instead, it is given on the right side of the posts page when you selected a category. Here a shortcut for the RSS for posts from the Blue Obelisk category.