I must try taking some action in php whenever someone posts a comment. How do i setup a hook for your? I suppose it might use functions.php...

EDIT - here's my udpated code:

add_action('comment_post', 'comment_posted');


    function comment_posted() {
        $MyClass = new MyClass(); //I do this in other functions and don't have any blank screen problems
            $test = "";
            $MyClass->doSomething($test); 
    }

See: http://codex.wordpress.org/Function_Reference/wp_new_comment

Calls 'comment_post' action with comment ID and whether comment meets the approval of WordPress.

You need to have the ability to then call add_action('comment_post', 'my_func'); from inside your functions.php.