Example of WordPress action for WordPress User Query filter on author of specific post type. https://gist.github.com/remcotolsma/6460846

gistfile1.php

<?php

$user_query = new WP_User_Query( array(
	'orderby'          => 'ID',
	'prefix_post_type' => 'blog'
) );

$users = $user_query->get_results();

foreach ( $users as $user ) {
	echo $user->display_name, '<br />';
}