Darkness is indeed the absense of reflection of light سے طرف کی definition, it's not so much what we know, but if dark is to be defined, then light has to be defined. Darkness is an adjective, and one might ask if آپ are talking about complete darkness. I would say the speed of darkness as I understand it would be the speed it arrives. The reason it is کہا that the absense of light is because light is a thing called eloctromagnetic waves.
Surely the speed of 'dark', which doesn't actually exist as others have already pointed out it is the absence of light, should be the speed a which it takes for the light to vacate the area that is 'dark'?
Or, if it is already 'dark', then there is no speed, unless آپ count the speed at which our eyes recognise that there is no light.