Black boxes are called cross overs. They take speaker input from the amp, and split the signal, high frequencies get sent to the tweeter, lows to the mid-bass cone. One wire goes in, two comes out. It gives you better clarity due to the fact that the speaker isn't trying to reproduce frequencies it was not made to make.
The Alpine has all of the following, but if you plan on getting something else I'd be sure to look at these:
If you plan on running the rears off the head unit and get a 4-way amp and plan to run the fronts of it and bridge two for the sub you will need to make sure that the amp is capable of being bridged. Some arent.
Also, if you plan on doing it this way you might make sure that it has varible hi/lo pass filters, you'd want a hipass filter on your outputs for the front speakers(blocks low frequencies from trying to be reproduced by your highs/midbass speakers), and a lowpass filter for the sub channel (vise versa). This again helps for clarity on the same principle of the componet speaker crossovers mentioned above.
And because I want to show off. This is my custom set up in my BMW I built. I used to work in a car audio/electronics shop in college.
http://forums.bimmerforums.com/forum/showthread.php?t=1421641