How do I bring the black back into a black fiberglass kitchen sink?
If it needs a good cleaning only I'd try Bar Keeper's friend. It works on fiberglass and a host of other surfaces. I have used it on my glass top stove and on a stainless steel sink, the results are great! You can usually find Bar Keeper's friend near the other abrasive cleaning products like Comet and Ajax.
The thing that works on my beige fiberglass shower is Dawn dishwashing liquid and a scrub brush. I don't see why it wouldn't also work for black fiberglass.
The sink may not be dirty. The surface might be worn and scratched. I had this problem in my old house and after I cleaned the sink and dried it, I waxed it with Johnson's paste wax. It has to be done often but you have to clean the sink often so it doesn't matter.
Bar keepers friend should work but if not get a product called Gel Gloss. You can also try and find the old Glass Wax. That will zip it right up.
Use LimeAWay or CLR. Dirt gets trapped against the acrylic painted surface. It all goes away, dirt and lime when you use it. Much better than scrubbing with something that can scratch the surface.