Black kitchen sink

https://www.thriftyfun.com/Cleaning-a-Black-Porcelain-Sink.html

Try using bon ami

Your water may be the cause. Try rinsing it with vinegar in a spray bottle. It removes minerals in the water. You might want a Pur filter on the faucet to get water without the minerals for the last rinse. We no longer have the problem now.

Clean thoroughly with water and a Magic Eraser. Rinse and dry with a soft cloth. Spray sink with a good quality furniture polish (no water ingredients). Next rub/polish with a dry soft cloth (microfiber cloth is good), polish until you see a shine. You will notice that water will run off and not spot. Your sink will be easy to clean. Polish your sink frequently, you will love it!

What's worked for me is bar keepers friend- good luck :)