I would like to have a second server blade that I have be used to display the message that my usual webserver is down. However, I'm not sure how to set it to only work when the other is down, otherwise it confuses the DNS.
Look at load balancers.
HAProxy, Varnish are some good software based LBs. In some instances, even web servers likes Apache and Ngnix can do load balancing. If you have the budget, you can look at hardware based load balancers.