“darkness” is just the absence of light. So the speeds must be equal, in a vacuum: 3×10^8 m s^-1,
imagine if that weren’t the case:
there would have to be some in-between stage of dark and light where one hadn’t caught up the other yet.
As light is travelling forward, then the darkness must be travelling backwards at the same rate. Therefore I would say that the speed of darkness is -3×10^8 m/s.
Has anybody asked Justin Hawkins how fast the Darkness is?