You can change the original string using any good editor (Notepad++, TextMate, SublimeText, NetBeans, coff... Dreamweaver), that will allow you to make a global search and find that string. Of course, this modifies the original theme and after any upgrade, you'll have to do it again.
One option could be: unregistering the meta box in your child theme and registering it again with the new name.
Two workarounds:
1) WordPress SEO by Yoast has the string properly registered and this works:
add_filter( 'gettext', 'change_text_wpse_81595', 10, 2 );
function change_text_wpse_81595( $translation, $text ) {
if ( $text == 'WordPress SEO by Yoast' )
return __('SEO','my-plug-text-domain');
return $translation;
}
2) The theme where I'm testing does not registers the string properly and jQuery has to be used:
add_action( 'admin_head-post.php', 'rename_meta_box_wpse_81595', 15 );
add_action( 'admin_head-post-new.php', 'rename_meta_box_wpse_81595', 15 );
function rename_meta_box_wpse_81595()
{
// Remove this if the change is to be made everywhere
global $current_screen;
if( 'page' != $current_screen->post_type )
return;
?>
<script type="text/javascript">
jQuery(document).ready(function($) {
// Locate the proper target, this one is just a sample
$("#woothemes-settings h3 span").html('Custom Box');
});
</script>
<?php
}
If you need to translate many strings, this may help: Change the text on the Publish button