I was mildly surprised that Blogger didn't seem to have captchas for comments like they do for account creation. Luckily I realized today that they do and I enabled it. Weirder is the fact that they hadn't enabled it by default.
Here's how to do it.
If you haven't done it yet, do so ASAP. Almost everyone I know who uses Blogger seems to have been spammed at least once in the past fortnight.