using hook_block_list_alter, programmatically hide/show a block on Views pgs and Node pages, based on if a flag field is set or not.

hook_block_list_alter_example.php

<?php
/**
 * Implements hook_block_list_alter().
 */
function hook_block_list_alter(&$blocks) {
  //dpm($blocks);
  
  //check if pg is node
  if (arg(0) == 'node') {
    $nid = arg(1);
    //load current node
    if ($nid) {
      $node = node_load($nid);
    }
  } 
  //views pg
  else {   
    $views_page = views_get_page_view();
    if (is_object($views_page)) {
      //get title of current view
      $view_title = $views_page->name;
      
      //get view
      $view = views_get_view($view_title);
   
      //get entity id from views header
      $entity_id = $view->display['default']->display_options['header']['entity']['entity_id'];
      
      if ($entity_id) {
        //load node of entity in views header
        $node = node_load($entity_id);
      }
    }
  }
   
  $field_value = $node->field['und'][0]['url'];
  //unset block if $field_value empty
  if (empty($field_value)) {
    unset($blocks[620]);
  }
}