According to Jewelry Notes, a black sapphire is a type of sapphire that has a nearly opaque appearance. While the stone itself is blue, it's so dark that it absorbs all light entering it, resulting in the appearance of a solid black stone.
Jewelry Notes explains that black sapphires come primarily from Australia, where they are mined in large quantities due to their considerable demand. Black sapphire is much more common and less valuable than true sapphire, and it is frequently considered a low-grade stone. There are various permutations of black sapphire that may appear as solid black, dark blue or even gray, depending on the way that light filters into the stone.
Because of the incredibly dense properties of sapphire, Jewelry Notes explains that black sapphire is often used as a more affordable and durable alternative to onyx. According to Wikipedia, the Black Star of Queensland is known as the world's largest gem-quality sapphire; in spite of its name, the gem is actually a regular blue sapphire. Jewelry Notes explains that black sapphire is also commonly mistaken for star sapphire due to its similar coloration. Star sapphire is actually a completely different stone that garners its unique reflective properties from a different crystal shape.