wp_blacklist_check
wp_blacklist_check is a 6 parameter (!!) action that gets passed a comment's information. It is fairly useless by itself, as nothing can be done with the data that is passed to it, so it needs to work in conjunction with pre_comment_approved. What you can do is make a decision about the comment in wp_blacklist_check and then immediately add a filter to pre_comment_approved to carry out your desired action (which should be 1, 0 or 'spam').
Context:
File: wp-includes/comment-functions.php
function wp_blacklist_check($author, $email, $url, $comment, $user_ip, $user_agent) {
global $wpdb;
do_action('wp_blacklist_check', $author, $email, $url, $comment, $user_ip, $user_agent);
This hook can pass multiple parameters. In order to get additional parameters (up to 1 for this hook) passed to your function, you will have to hook in like this: add_filter('wp_blacklist_check', 'your_function', 10, 1); where 10 is your function's priority, and 1 is the number of parameters you want your function to accept. Note that your function should only return the first parameter.
This hook is an action which means that it primarily acts as an event trigger, instead of a content filter. This is a semantic difference, but it will help you to remember what this hook does if you use it like this: add_action('wp_blacklist_check', 'your_function');