$delta of the block

Just wondering why the $delta of the block is 'form'. This seems overly confusing when trying to alter the block.

This is what I did to programmatical set the title of the block to none.

function hook_block_view_textsize_form_alter(&$data, $block) {
  // Set the title of the block to none.
  $data['subject'] = '<none>';
}