Recently Google discontinued reCAPTCHA v1(see more here.) and the only plugin on Blogs for reCAPTCHA never updated to reflect this change.
So in walks our simple plugin replacement aptly named Simple Google reCAPTCHA. The plugin only requires that you sign up for reCAPTCHA and follow the steps to acquire a site key and secret key that will then be used in the plugin’s settings(Site Dashboard > Settings > reCAPTCHA). Once this step is done, reCAPTCHA will be used on all comment forms, that is if the user is not signed in, before the user can submit a comment.
What is reCaptcha? Learn more here.