Load .js only if block is loaded
Depends on what type of block your need the js for. If it's a custom block you created in a module, you can just tack drupal_add_js into the block definition function. 

If your trying to add it to a block was created by someone else's module, or drupal created (IE, you didn't create it with a module), you could create a simple module that uses hook_block_view, which is fired whenever a block is loaded. You should be able to check from there if that is the block you want, then use drupal_add_js to load your js from that module.