Alter a form in a block only if it is in a certain region
use this code in hook_form_alter:

function cmfzen_form_alter(&amp;$form, &amp;$form_state, $form_id) {
    switch($form_id){
         case 'views_exposed_form':
         $region = 'NAME_OF_A_REGION';
         $blocks = block_list($region);
         kpr($blocks);
         //if the block is in the $blocks array do the changes.
         break;
    }
}


using block_list($region) you will get the blocks that are placed in a specific region, and it returns array of block objects (for more detail visit this page). Now check if the block you are looking for is in the $blocks array.