Is there a place to post your back-cover blurb for critique?

Yes. But you need to have 50 posts first. It's the Share Your Work forum, specifically Query Letter Hell. The password is vista. While you are getting your posts up to 50, you can still go there and read the stickies, look at other people's blurbs and queries, critique others or just look around to get familiar with the culture of crit rooms.