Replace/alter/rename form labels in theme
Solved!

Put this code to your theme's template.php, where's THEME replace it with your theme's dir name. So then where's #title = t() you put in your own custom title for the field. So what it does it alters given forms field, in this case it's simplenews_block_form inlcuded in between THEME_form_simplenews_block_form_alter

function THEME_form_simplenews_block_form_alter(&amp;$form, &amp;$form_state) {
  $form['mail']['#title'] = t('Join Our Mailing List:');
}


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