Yesterday, forgetting about the template problem, I blithely typed up and published a new post. This action republishes the entire blog -- replacing the current page with the corrupted code, and blinking my beautiful blog mostly out of existence, with only a few hideous scraps of html remaining to show it had ever been.
I sent an e-mail to Blogger for help, but I doubt they keep any backups. I was hesitant to make any updates, lest I lose any chances of the old template being restored. But when I got home (I made the ill-fated new post from my father's house, which may account for my absentmindedness), I was delighted to see that I'd left my computer in hibernation as usual, and an old copy of the blog was still open in my navigator window. I copied and saved the page code, and used it to reconstruct my template, creating a new test blog for the purpose (http://bluewyvernteatest.blogspot.com/ if you are for some reason interested -- I think I'll keep it around for emergencies). Deciding there was no point in waiting for help, I replaced the Blue Tea template myself, and hoped that the support people might at least tell me what happened and prevent it from happening again.
And then there was another, totally unforseen problem -- when I went to view my blog, I got the baffling message that "The file / cannot be found. Please check the location and try again." It works fine in the preview, and the test blog is still there, but for some baffling reason, Blogger no longer likes my domain name. If you're reading this, the problem has been resolved somehow, but gosh, I sure don't know.