Index: wp-includes/comment-template.php
===================================================================
--- wp-includes/comment-template.php	(revision 18190)
+++ wp-includes/comment-template.php	(working copy)
@@ -307,12 +307,17 @@
 	$classes[] = ( empty( $comment->comment_type ) ) ? 'comment' : $comment->comment_type;
 
 	// If the comment author has an id (registered), then print the log in name
-	if ( $comment->user_id > 0 && $user = get_userdata($comment->user_id) ) {
-		// For all registered users, 'byuser'
+	if ( $comment->user_id > 0 && $user = get_userdata( $comment->user_id ) ) {
+		// For all registered users (site-wide), 'byuser'
 		$classes[] = 'byuser';
 		$classes[] = 'comment-author-' . sanitize_html_class($user->user_nicename, $comment->user_id);
+		
+		// For current blog users
+		if ( ! is_multisite() || is_user_member_of_blog( $comment->user_id ) )
+			$classes[] = 'by-blog-user';
+			
 		// For comment authors who are the author of the post
-		if ( $post = get_post($post_id) ) {
+		if ( $post = get_post( $post_id ) ) {
 			if ( $comment->user_id === $post->post_author )
 				$classes[] = 'bypostauthor';
 		}