Sounds like it could be the closed captioning.
Check the CC options on your TV.
Some STB's also have CC options in the menu, check menu > settings > accessibility.
2500's don't have this accessibility menu as it just passes CC to the TV.
I have the same problem. I have turned off the closed captioning on the STB and the TV. The black box pops up only on certain channels - mostly while watching football games. It won't go away until I unplug the stb and switch to a non sports channel. I called tech support and they reset the set top box. But the problem still exists. Any suggestions?