Blackouts in Europe don't happen because most right-holders are national.

But basically, a particular sports event shown on one channel will not be broadcasted because another channel holds the rights in that juridiction.

It happens a lot on Sattelite TV in Africa...

TF1, the biggest French Channel is broadcasted on Canal Sat Africa. TF1 shows Formula One events. But they only hold the rights for France.

Since Canal Plus (the channel) or SuperSports (South African Channel) hold the rights for Africa, TF1 is blacked out during Formula One events.