So I did a little investigating :) And it turns out that it's a known Blogger glitch.
A CAPTCHA is a security setting that requires the commenter to "Please prove you're not a robot" by keying in the sequence of letters and numbers displayed in the image above the textbox. This is called "Word Verification", and can be turned on or off by the blog administrator, according to preference. The idea is to cut down on spammy comments. A typical Word Verification box looks like this:
In this particular CAPTCHA, which I saved a few months ago, I couldn't read the number part at ALL, hence the question marks :) Thankfully you can reload the images (just click on the Refresh circle arrow to the right of the box), but to me they seem to be getting harder to read! The new Number Verification looks like this:
However, the problem is that it is the result of a glitch in the Blogger Code from a recent Update. The full discussion in the Blogger Help Forum is here, and Chuck Croll, who advises in the Forum as "Nitecruzer", has written a must-read post about it here on his blog, The Real Blogger Status, which will be updated with new info ASAP.
Basically, if you have your comments set to pop up in a new window - like I do - instead of embedded at the bottom of your post AND do not moderate your comments (I do not allow anonymous comments, but otherwise I don't moderate), this new CAPTCHA is automatically added to your Comments form and turning OFF "Word Verifcation" will not remove it.
Unfortunately, the Number Verification CAPTCHA is a fake - you may be able to ignore it entirely and publish as normal, so it's not actually a functioning security feature! You can change to embedded comments to make it go away for now, or check your Cookies following the steps suggested in the posts linked above.
This is a minor change, and Google is always busy, so I wouldn't expect a fix anytime soon. My issue is not with the glitch itself, it's with the lack of communication from Google about it. It's scary to open your Dashboard and find a new feature you didn't add - especially when it doesn't even work! - and it can cause real panic when you discover that you cannot remove it. That's one of the signs of a hacked blog, after all!
It may also be confusing to readers trying to leave a comment! And if you're trying to comment on multiple posts (like, say, catching up with your blog reading) it may be a little bit tedious to keep typing numbers, although at least they aren't letters LOL ;)
So please share this information with other Bloggers you know, in hopes that any misunderstanding or frustration can be avoided :)