I was wondering how to add superscript characters like trademark to Drupal block title.
Block Class Styles may be useful:
Extends the Block Class module to incorporate styles (or themes) rather than css classes. Adds style-based tpl suggestions. Allows HTML in your block titles.
Other options would be to create your block in code, and set the
$block->subject to be your
<sup> inclusive title, or to override the
block-XYZ.tpl.php for the your particular block and add the title in there.
By default, Drupal core runs
check_plain on any UI created blocks, so through the UI I don't think there's another option.
Trademark ™ is just a character, it doesn't need html. There is no reason why you can't have special characters in block titles.
Superscript as in sup tags is a different story, see the other answer.
How to type symbols? http://www.wikihow.com/Type-Symbols-Using-the-ALT-Key