If you shine a flashlight at an observer and then turn it off, the “darkness” will reach you at the speed of light. You could say that the last photon from the flashlight will reach you in c*d, where c is the speed of light and d is the distance from the flashlight.
We definitely have an idea of what the speed of light is. It’s exactly 299,792,458 m/s.
Asking what the speed of darkness is is like asking what the kinetic energy of coldness is.
The question is based on a wrong idea: that light and darkness both exist and are somehow opposites, i.e. “dualism.” Darkness is not a thing. Coldness is a thing. Darkness is the absence of light; coldness is the absence of heat. Both heat and light are forms of energy.
If I 50% pulse width modulate a light beam at 300 MHz, I will make packets of light about one meter long separated by darkness a meter long. They both travel at the same speed – “c”.
Nobody knows…. it’s never moved… but there are two scientists in Sweden that have been standing by with a Radar Gun since 1985 staring at darkness just waiting for it to move.