Why doesn't Drupal use my block.tpl.php? I want my Drupal 7 theme to use a custom block.tpl.php for my navigation bar (Defined: regions[navigation] = Navigation bar). So I copied block.tpl.php from the block module to my theme directory and tried to give it different names (following different guides on the web). None of them seemed to work. 
Then I wanted to make sure it was a naming problem so I changed the name back to block.tpl.php and changed its contents (removed the h2-tag for the title). I reloaded my site but nothing happened so there must be another problem.

Any ideas on how to fix this?

Thank
Most likely you just need to clear Drupal's caches. 

Template file implementations are cached in the theme registry so you need to clear it for new files/theme functions to get picked up.

You can do so either at http://mysite.com/admin/config/development/performance or by using Drush.