Have you added the language string to blocks/MYBLOCKNAME/lang/en/block_MYBLOCKNAME.php ?
(i.e. $string['pagetitle'] = 'The name of this page'; )
After that, have you purged all caches (site admin > developer > purge all caches)? (Or you can bump the version number and run an upgrade, but purging caches is usually more convenient during development).