How to clean a toilet of black streaks?

Sounds like mold. I would check the tank, and put vinegar in there daily until its gone. Get some Bar Keeper's Friend and scrub out the toilet.

Vinegar works well, but this sounds like a job for bleach. Just pour a small amount in the toilet and let it stay for at least 30 mins. Do not ever mix cleaning products, so if this does not work wait a few days and try something else.

Bleach will not be effective to rid this situation. You have to put white vinegar and baking soda in the tank and the toilet and allow to sit over night. If that does not work use a pumice stone.Here is one other option. https://www.hometalk.com/diy/clean/bathroom/toilet-cleaning-bombs-30029319

The vinegar will kill the germs, mold and mildew spores, but will not do the work on the stain like bleach. It should be used after you use the bleach on the stain.

Get a sandpaper sponge. It works great to remove any stains in a toilet

Get a pumus stone and scratch off the stain, after cleaned put a sanitizer in the tank from Walmart called The Works. Just drop in your tank and it’s good for 30 days. We have hard water here in Arizona and this keeps the stains and calcium away.

Flush the toilet and then pour a quarter cup of Borax into it. Use your toilet brush to swish it around. Next, add one to two cups of vinegar and let the mixture sit for 30 minutes. You should be able to scrub the stains away!

I soaked paper towels with bleach and patted them down over the stains. Let sit over night a rinse. Depending on how bad the stains are you might need to repeat this step. Follow up with vinegar that will kill the mold spores but the bleach lightens them.

Here's a post that might help - https://chascrazycreations.com/easy-and-weird-toilet-cleaning-hacks-that-work/