drupal-comments-prevent-spam-recaptcha-module
The recaptcha module requires downloading, installing, and creating a "recaptcha user" on their website to get a site key (for a domain or many sub domains)
http://recaptcha.net/api/getkey
administer -> user management -> ReCAPTCHA (tab) fill in the Public Key and Private Key from your previous registration above
administer -> content mangaement -> content type -> "edit blog entry" allows you to do some basic modification of who/how comments are added (nothing to do with captcha yet)...