There could be a few different reasons that an event is in a blackout period.
If a school is broadcasting their own game, they reserve the right to blackout the on demand replay for a period of time of their choosing. The most common reason for a school blacking out their own game is so that other schools cannot study their formations and plays to gain a competitive advantage.
A game can also be blacked out due to conflicting television and media contracts. There are also numerous local and regional ordinances on the books concerning high school athletics that must be adhered to.