@thorninmud has it right. It is the absence of light, so if someone put a black sheet in front of the sun, the dark would hit us at the speed it takes for all the light ahead of it to reach us, and then not reach us.
This reminds me of the amazing spectacle I saw yesterday as I waited for the bus. The light at this time of year is really special up here at 63 degrees north. I saw these streaks of ‘dark’ in the sky and I couldn’t work out if it was my eyes playing tricks or if they were coming from somewhere and what they would be coming from, in that case. It was between noon and 1pm. I realised, if it was something, it had to be the sunlight and it had to be blocked by something. Examining the sky, there were clouds. The angle of the sun in the sky was producing shadows that were so LONG and ANGLED that they appeared like long, unnatural streaks in the sky, some making landfall across the fjord and some making shadows beyond where I could even see. It really was something.