Mirror site basically allows you to add your domain name within dreamhosts namespace. It means if you are on the dreamhost network its accessible to you. But even as customers we aren't actually on that side of the wall.
One possible method, is to reconfigure your computer to use dreamhost's nameservers (instead of the default supplied by your isp), and/or add a temp entry to your computers HOSTS file. The limitation with that method is that it is a single computer solution.
With a mirror basically what you do is establish blog.domain2.com on dreamhost network, but the DNS doesn't yet point to that space. Add a mirror and it's essentially clones the DNS so that you can access that under another name.
With either your method or with the mirror method, you still have to watch how links are created. Or moresimply when you are ready to change the DNS just do a search and replace in your MySQL database via phpMyAdmin. you would be searching the mirrored temporary domain name and replacing it with the permanent domain name.