If there is a speed of light, is it possible that there is also a speed of darkness?

well, seeing as darkness is simply the absence of light, i might want to say that the speed of darkness would be equivalent to the speed of light, or rather the speed at which light can be removed from the area in question. this would be a bit of an abstraction though. Light has actual material (material being different from mass) that composes it, which is why it is allowed a characteristic such as speed to be applied to it. darkness however (unless we're talking about wavelengths of light that are outside the visible spectrum, which i dont think is the case) is the absence of any light. Makes for a difficult question. Scientifically, no, there is no speed of darkness because it doesnt travel. "it" doesnt even really exist. but for the sake of conversation though, there may be non-scientific ways you could argue that there is a speed of darkness.

You need to understand physics a bit better. "Speed" is defined as the ratio of the displacement of an object and the time required for the displacement to occur. "Object" is loosely applied here. Light itself isn't a massive object but instead pure energy in the form of a wave. It still is an object of some kind however. Darkenss is the absence of light and you are asking how fast nothing travels. I suppose the answer is zero, because nothing is there in the first place so the displacement of nothing is also zero. Nothing does not move. Ever. I'm afraid this whole question is an exercise in pure semantics more than anything else.

It is actually more accurate to say speed of darkness because the speed is based on electromagnetic radiation. Visible light being just a tiny tiny part of the total radiation. However, when you consider the space that occupies the radiation as a dimension/quantity the former definition is non applicable.

it would be the same speed

darkness is the lack of light

light travels thru darkness at a set speed

the speed of darkness would be how fast light left the area = speed of light

So using same logic:

if there exists the "speed of a train", is it possible to have "speed of a non-train"?