How do I send an email blast from Mail?

You create a group in Addressbook of all your contacts you want to mail.
Drag said group into the BCC field of your compose new message and not the CC nor To field. If BCC isn't visible, I think the View menu allows you to enable it when composing a message. This way you preserve their privacy, do not increase the chance anyone will be spammed, and send to all of them at once. There is one caveat. Your internet service provider may have a cap on the number of outgoing messages that can go out at once. Check that before attempting this, and decrease the number of people in your group accordingly.

If you intend to handle creative content and distribution in house, you should take a look at using Direct Mail from e3 Software. It is powerful, inexpensive, easy to use and well maintained by the developer.

I believe that this is a far better approach to the challenge that you face than using a mail client like Mail.

Thanks Michael, I was having the same problem, found this thread, and just tried e3 mail merge, just what I was looking for!