Is there a black unicorn?

Could you please elaborate on the concept of a "black unicorn"? Is it a mythological creature or a metaphor for something? Given the rarity and mythical nature of unicorns in general, is there any scientific or cultural basis for the existence of a black unicorn specifically? Have there been any reported sightings or legends surrounding this enigmatic creature? Additionally, are there any particular cultural or religious connotations associated with the idea of a black unicorn?