Im not entirely sure thats correct. Change this around a bit its what i use for bold text
$bold = "/([B])(.+)([\/B])/";
so change that to
$quote = "/([quote])(.+)([\/quote])/";
Now i havent tested it with the change but as its virtually the same as bold it should work.
The do this
with the \2 just have what you wanna use. Most likely blockquote
$message = preg_replace($quote,"\2",$message);