Facebook has started throwing a CAPTCHA prompt for all Google Blogger blog links. This means that when you want to share a blogspot.com link, or any link hosted by Google Blogger, Facebook won’t immediately let you do so. Instead, you’ll be prompted to verify that you are certainly a human and not a bot.
That's interesting. Perhaps they've been getting too many spam drops. I'm surprised it isn't normal for CAPTCHA's on FB with any link. Perhaps many wouldn't share if they were presented with a CAPTCHA each time.
Funny, I just posted a link to blogspot minutes ago and didn't get a captcha.
Maybe they verify your blog only once or maybe there's some of criteria triggering the captcha, such as a first time link dropper to blogspot or whether people have complained about your links in the past.