ATV (black) software version - latest - CAN NOT USE BETA YET

It is likely that your ATV (black) has the latest version of Apple software on it  This is called 4.1.1 or sometimes 4.2.1 .

You will not be able to jailbreak (the first step) to use ATVflash-beta on your ATV(black).

In fact most have upgraded to Apple software latest, because you can not download a movie to your apple tv and have it play, without the latest fix to the software.  Found this out recently, and allowed apple TV to update.