How do I get rid of black streaks in a white enamel bath tub?

Steamclean or try bleach

The enamel on steel or cast iron tubs does wear thin over the years.

If that’s whats happening, then look at the tub re-coating products our there & re-coat the tub. They have a tendency to drip or run, so I’d follow the directions to a T.

BTW, I paid a Jacuzzi factory-certified technician to re-coat my two person Jacuzzi spa and their job dried with a run or drip in it. Not good 😣

I redid my kitchen sink with a kit made for tubs. Click on the picture next to my name, here, and it will take you to the project.

If the porcelain is coming off and you're seeing the cast iron beneath you'll likely have to have it reglazed. There are companies that will do it, but there are also many DIY Options online - https://amzn.to/2pnls8z

Paint with tub and tile paint kits

https://www.hometalk.com/search/posts?filter=paint%20bathtub