[SOLVED] Validation of block config settings in Drupal 7

Been struggling with similar myself and what I've come across is that this doesn't seem to be possible because the block configuration uses a separate hook_block_save().

If you wish to add validation for the whole form, the best way I have managed to implement myself is to add a custom function to the code with $edit as a parameter and a boolean as a return value. Then call the function in the hook_block_save() before saving data in order to perform the extra checks.

Works fine with, you can set form_set_error in the custom function etc, the only exception is that it will not highlight in red values that are incorrect in the form. Also, it will still try to redirect to the blocks listing page but this can be bypassed by adding a drupal_goto() to the custom function.

This is the code I wrote, and it works just fine for me.

/**
 * Implements hook_block_configure().
 */
function mymodule_block_configure($delta = '') {
  if ($delta == 'myblock') {
    $form = array();
    $form['website'] = array(
      '#type'          => 'textfield',
      '#title'         => t('Website'),
      '#size'          => 60,
      '#required'      => TRUE,
      '#default_value' => variable_get('mymodule_myblock_website', ''),
    );
    return $form;
  }
}

/**
 * Implements hook_form_FORM_ID_alter().
 *
 * Adds validation of block configuration custom fields.
 */
function mymodule_form_block_admin_configure_alter(&$form, &$form_state, $form_id) {
  if ($form['module']['#value'] == 'mymodule') {
    $form['#validate'][] = 'mymodule_block_validate';
  }
}

/**
 * Form validation callback of block configuration.
 */
function mymodule_block_validate(&$form, $form_state) {
  $delta = $form['delta']['#value'];
  if ($delta == 'myblock') {
    if (!valid_url($form_state['values']['website'], TRUE)) {
      form_set_error('website', t('The value provided for %field is not a valid URL.', array('%field' => 'Website')));
    }
  }
}