Anyhow, it was obviously a considerate hacker because they didn't do anything malicious (that I'm aware of yet) but left a nice message instead:
Your Blog Has Been Hijacked
Don't use this method (cross-posting from Google+) to post to your blog because it reveals your secret email address that allows you to post content on your blog. If I was a malicious person, I could post some very bad things here. Please change your secret email address for posting.
Thank You, and have a nice day!
Nice, and thanks!