What should I do about a cyst that feels like it has a sack of liquid on top and is turning black on my face?

See MD. You should see a doctor as soon as possible. This could be serious and disfiguring. See a pcp, dermatologist, or other doctor. I would not hesitate to be seen.
Facial cyst. Please see a board-certified dermatologist or plastic surgeon to evaluate your condition. If it is a cyst, sometimes only surgery is the best option to remove the growth, but this needs confirmation. There may be alternative non surgical treatments, but please don't squeeze the growth as this may cause permanent scarring if it leads to dermal rupture and infection.