block.tpl.php is the only template that matches all blocks, but the only thing it effects is the wrapping html. This is good if you want to theme blocks to have round corners, or stuff like that. The actual content of the block will not be effected by this.
Since modules can render a block in many ways, you can't really create a single template file where you can handle how the output should be. If you want to alter the output of many different blocks, the only way to go it to handle them on a case by case basis.
You should be aware that CSS goes a long way and you might not need to alter any output or use any template to theme the blocks, if you write the needed CSS.