There are plenty of synonyms for the phrase "dark sky background". Apart from describing it as dark or black, words like obsidian, onyx, ebony, pitch-black, midnight, and coal black can be used too. The term sable can also be handy, referring to the fur of animals that's black or dark brown. Jet black is another synonymous phrase often used to describe a deep and dark hue that resembles the color of coal. Other descriptions for the phrase "dark sky background" include ink black, coal dust, shadowed midnight, and charcoal. By using synonyms for the word "dark sky background", you can add color and variety to your writing.