Well if you want to use different urls like subdomains you simply change the path to your style sheet in the head tag of your header.
<link rel="stylesheet" type="text/css" media="screen" href="red.css" title="default" />
in the red.myblog.com
<link rel="stylesheet" type="text/css" media="screen" href="blue.css" title="default" />
in the blue.myblog.com
You probably have your good reasons to do that, however if you want to simply offer 2 styles you can avoid the 2 urls. you can simply use a button with the javascript.
There are probably better exemples but the first that comes to mind to me is http://klubitus.org/kalenteri.php? on the top right it let's you choose among several color to switch color scheme. simple and effective.