I am indebted to the Old Curmudgeon for this public service information. It's a way round the Ion curtain that Blogger have thrown up to separate the Betas from the rest of us.
Before you write your comment, click on other. A different comment box will appear with a space for your blog handle and your URL. Enter these, then type in your comment. Now you can post the comment, and it will appear in the list of comments on the beta blog. The only problem is that your name appears in black, indicating that readers of the blog will not be able to link to you by clicking on your name, so it ain't a perfect fix, but it'll do for the time being until Blogger see fit to undo all the chaos they have sown.