Light is the absence of darkness. Conversally darkness is the absence of light. You can remove darkness by shinning a light, and then suddenly turn off the light. The speed at which darkness proprogates through the room will indeed be the speed of light.
Darkness cannot move slower than light because where darkness hasn't moved to yet there would still be light. Light cannot move slower than darkenss for the same reason.
Light and darkness are both vectors (direction), so the velocity (also a vector) of darkness would have the same direction (vector) as light.
Oh and great work for Bintel and Aussie Post.