black color or no light reflection, at day time u see black color but at night same color seems to disappear. I mean what color really sky is. is it blue, black white, or colorless or has so many different colors that our eyes even cannot identify. 

SO, dark means the light has not been reflected from the object or our eyes cannot get rays from that object. Does dark has speed, yes infinity, thats y we do not see rays reflected from dark ( object). Again, dark means that our eyes does not have enough powers to see them or the rays reflected from object cannot get to our eyes. 

i am guessing dark refers to black color.