How do I clean off Black toilet bowl crust?

Scrub with a pumice stone from hardware store. Wear rubber gloves.

I would get every bit of water out of the toilet and then spray very generously with Lime-Away or CLR onto the dry surface. Keep it moist and check it every 15 minutes to see if it is beginning to loosen. Apply more of the cleaner if it’s drying out. A 3M stiff green scrub pad would be a good flexible option that would get into curves. Heavy duty rubber gloves are a must. If the stain could be an iron buildup, then IronOut is worth a try.

https://home.howstuffworks.com/home-improvement/household-hints-tips/cleaning-organizing/how-to-clean-mineral-stains-from-your-toilet.htm

Once upon a time I used a long-handled flat head screwdriver and very carefully scraped away at it.

It took a bit of elbow grease and determination, but it all came off and I didn't damage the bowl.

Pumice stone

Pour white vinegar and bicarbonate soda into the bowl leave for a little bit then scrub with the loo brush and flush. Might take a few times but it should help

white vinegar, dawn, and baking soda. Empty out the bowl of water, spray, and then place a paper towel over to keep wet. Let sit for several hours. Then clean with a scrubbie or a toilet brush. Be sure to take the paper towels out, do not flush them.

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 emptied my toilet bowl and used a steam cleaner. Got it clean but a lot of work.

The Works toilet bowl cleaner. Original thin one

Here's a post that might help - https://chascrazycreations.com/how-to-clean-your-toilet-from-top-to-bottom/

vinegar

A denture tablet or two in the bowl will loosen the gunk and then you can clean it out with a brush. I have heard that real Coke Cola works wonders too.

Hello Miguel, hope this helps you.

Vinegar and baking soda: Add 1 or 2 cups of vinegar to the toilet bowl along with a few sprinkles of baking soda. Swish the solution around the bowl with your brush for a few minutes and then let it sit for about 15 minutes. Scrub the stains with your brush (or pumice stone). Repeat as necessary

Turn the water off to the toilet bowl and flush so no water remains in the bowl, then pour in CLR, a liquid looking product you can purchase most any where cleaning products are sold. Let the CLR sit in the rusty area or soak a paper towel in it and apply to the rust areas. Let set for as long as possible and scrub with a toilet brush. Turn the water to the toilet back on and flush. You'll see great results!