Sometimes the usual rules don't apply, and such is the case when hooking the module computed_field.
The standard Drupal comment approval admin page has always been an annoyance to me. The main reason I will toss a comment is that it's spam. You know, like "I love this blog. For a great 3-day cruise..." and the like. The problem with the approval page is that it shows everything BUT the comment.
I created a Drupal 6 module, and in it I implemented hook_block(). I set up the block to appear in a certain region, and hook_block acted like a blockhead. The block consistently appeared in the Disabled list.