same as the speed of light, it would have to be else it couldn't keep up and when somebody turned out the lights there would a little gap of nothing while you waited for the dark to come along.
The speed of darkness is infinite. The shadow of the moon moves arcoss the face of the earth at infinte speed as it leaves the earth at the end of an eclipse.
Darkness can travel faster than light because darkness has no mass. Anything with non zero rest mass becomes infinitely heavy at the speed of light.