I've had messages frequently telling me they couldn't post comments to my blog (which upset me really, as I like seeing comments on my blog!). Well, I think I may have worked something out, although it may not apply to all.
Just tried to post on Talli Roland's blog and I'd gone via the Network blogs link on Facebook. Everytime I ever click on 'post a comment' the screen goes blank. It's done this to me with other blogs I've connected to via Network Blogs, not only Talli's.
I have to click on Talli's website, to close down the network blogs bit (or click on the 'close' which appears in the Network blogs bar above my own blog) and then I can post a comment.
Anyway, give it a try... see if that maybe the problem. You can use this blog post as a test ;-)
If not, not sure what to suggest. It must be blogger being temperamental at times. I think everyone can post, only anonymous can't. I did that to stop the Spam I'm afraid.