Form validation handler for block_add_block_form().
function block_add_block_form_validate($form, &$form_state) {
$custom_block_exists = (bool) db_query_range('SELECT 1 FROM {block_custom} WHERE info = :info', 0, 1, array(
':info' => $form_state['values']['info'],
))
->fetchField();
if (empty($form_state['values']['info']) || $custom_block_exists) {
form_set_error('info', t('Ensure that each block description is unique.'));
}
}