Try a 50-50 solution of bleach and water on black. If it works at all you can increase bleach strength. Be sure and rinse well with water before switching to another product.
Try plain white vinegar, warm it soome, almost hot, let it soak for a while.
You know not to use abrasives on glass, Bar Keepers Friend and Zuds are low abrasive chemicals with bleaching action. Use a nylon srubber. If you need to scrape, try cheap throw away putty knives, wooden kitchen spatula, popsicle sticks.
If black seems to be glue or caulk residue, mineral spirits ( paint thinner, NOT paint REMOVER!)), products such as 'Goof Off" "Goop" may remove it, if label says safe for fiberglass ( most are) and that it does work on glues and caulking.
You may need to go to stronger chemical. Boating, marine supply may be better source for them. Try a stronger oxalic acid product than Bar Keepers.. They can suggest other products. Work your way up from bleach and vinegar to stronger stuff.
The "etching" may be more of problem, how deep is it? Not thru to glass substrate? Some folks have had luck hiding slight pitting with silicon spray, which will need refreshing from time to time. Some use ordinary paste wax. Marine supply will have various fillers, "waxes" and "polishishes" and finish refreshers which are more permanent, thicker, some colored. There are 'artificial gel coats" for small areas.
You may have to go all the way to two part gel coat, which can require light sanding and buffing to get it to blend in. The worst case scenario would be complete refinishing.
[CENTER][FONT=Comic Sans MS]Measure twice, cut once.[/FONT][/CENTER]
[CENTER][FONT=Comic Sans MS]Look at the nail, not the hammer. Watch the fence, not the blade.[/FONT][/CENTER]
[CENTER][FONT=Comic Sans MS]If you hook your thumb over your belt you won't hit it with the hammer or leave it layin on the saw table.[/FONT][/CENTER]