In an effort to get rid of spam accounts signing up, and therefore flooding our emails with "New member" emails, Blaze added a SMF plugin for reCAPTCHA. I personally like reCAPTCHA, it's the only CAPTCHA that I really like.
If you know anybody who's having issues signing up, let me know and I'll see what I can do. I had to hardcode reCAPTCHA's ip in my firewall to make it work, which may not be the best way to do it.