Coal-black adjective – Of the blackest black; similar to the color of jet or coal. Stygian and coal-black are semantically related. In some cases you can use "Stygian" instead an adjective phrase "Coal-black".
Stygian adjective – Being without light or without much light. Coal-black and stygian are semantically related. Sometimes you can use "Coal-black" instead an adjective "Stygian".