Surfing the net, it soon became clear that this was a widespread problem. The general theory was that Blogger's spambot had run amok and returned numerous false positives.
Blogger Buzz admits to the problem as follows:
The good news this morning is that I am allowed to post again, but the episode has brought home to me that I rely upon Blogger but have no control over its actions.
While we wish that every post on this blog could be about cool features or other Blogger news, sometimes we have to step in and admit a mistake.
We've noticed that a number of users have had their blogs mistakenly marked as spam, and wanted to sound off real quick to let you know that, despite it being Friday afternoon, we are working hard to sort this out. So to those folks who have received an email saying that your blog has been classified as spam and can't post right now, we offer our sincere apologies for the trouble.
We hope to have this resolved shortly, and appreciate your patience as we work through the kinks.