Just spent over 1hr on technical support troubleshooting an error with a black screen covering the picture on one of my tvs with a standard box. Symptom: TV screen shows a large black square that covers most of the screen. After hitting the menu button on the tv, the black square would disappear but only momentarily. Also when initially turning on the tv the black screen is not present. The technician walked me through multiple resets in an effort to troubleshoot. After the last attempt, he instructed me to wait 30 minutes while a full reset of the box ran and by the way also check the status of closed caption. Resolution: After turning the closed caption option off the black square covering the picture disappeared. Issue resolved, it's a good thing that the closed caption option is a luxury and we do not have any deaf members of our household.
Sounds like you had your Closed Caption decoder set to one of the TEXT modes. This is normally not used for closed captioning, but it is sometimes used for extra information, such as TV schedules and what not (hence the large black square).
If you want to look at Closed Captioning, it is usually on CC1. SOMETIMES CC2 is also used, for alternate languages.