I had several troubles with MT-blacklist for example completely driving the commenting stuff unusable because of weird HTTP-500-errors.
Had to completely remove my whole MovableType directory on the server and install a full MT installation instead of an update and bringing back the configuration files from a backup.
Re-installed all plugins and now commenting is working again somehow.
You'll still receive HTTP-500 errors if you comment but the comment is inserted nevertheless. At least one can express their thoughts now.
Update: I also get HTTP-500 error after posting a new blog entry but post is created correctly. Strange...