Installing Blackbox is a simple process. You will want to make sure that you have selected the alternate install repositories.
Open a terminal window and use the following command:
sudo apt-get install blackbox
Once the install is complete, log out and click on the Options button in the lower left hand corner:
You will see a menu listing all of the window managers installed on your system. Select Blackbox from the menu, and continue to log in.
The Blackbox desktop is extremely simple, and looks a lot like Fluxbox. Right clicking on the background will give you the menu:
Really simple stuff. For more information about Blackbox including configuration tips, check out the home page.