Can't preprocess my search form

I'm using blueprint for the first time (switched from Zen -- like it so far!), but it seems that now my search block preprocess function is not working at all.

I'm using:

function blueprint_preprocess_search_block_form(&$vars, $hook) {
// Modify elements of the search form
unset($vars['form']['search_block_form']['#title']);

// Set a default value for the search box
  $vars['form']['search_block_form']['#value'] = t('');

// Add a custom class to the search box
// Set yourtheme.css > #search-block-form .form-text { color: #888888; }
  $vars['form']['search_block_form']['#attributes'] = array(
  'onblur' => "if (this.value == '') {this.value = '".$vars['form']['search_block_form']['#value']."';} this.style.color = '#000000';",
'onfocus' => "if (this.value == '".$vars['form']['search_block_form']['#value']."') {this.value = '';} this.style.color = '#000000';" );

// Modify elements of the submit button
unset($vars['form']['submit']);

// Change text on the submit button
$vars['form']['submit']['#value'] = t('Go!');

// Change submit button into image button - NOTE: '#src' leading '/' automatically added
//$vars['form']['submit']['image_button'] = array('#type' => 'image_button', '#src' => 'sites/all/themes/yourtheme/images/search-button.png');

// Rebuild the rendered version (search form only, rest remains unchanged)
  unset($vars['form']['search_block_form']['#printed']);
  $vars['search']['search_block_form'] = drupal_render($vars['form']['search_block_form']);

// Rebuild the rendered version (submit button, rest remains unchanged)
unset($vars['form']['submit']['#printed']);
$vars['search']['submit'] = drupal_render($vars['form']['submit']);

// Collect all form elements to print entire form
  $vars['search_form'] = implode($vars['search']);
}

Am I missing something obvious here?

Thanks.