Do you ever wonder why you get that black ring in your toilet and at times a streak of black coming from under the rim? It’s not the water nor the bottom of the rim that cause this.  Here’s the answer.  It’s the internal of the tank specifically the seal that the flapper closes on.  This black gasket is where the flapper rests and opens in order to allow flushing and closes so the tank can refill.  As with any gasket, it deteriorates after time thus the black material. Here is the simple cure to stop black ring in the bowl.
1. Close the water inlet valve
2. Flush the toilet holding the flapper open in order to drain as much water as possible
3. Using a cloth that you will dispose of later, gently wipe around the black gasket as you
hold the flapper open. Do this a few times. DO NOT USE HARSH CHEMICALS OR SOAP    NOTE: It is recommended that you wear rubber or latex gloves when cleaning the gasket
as the black does not clean off easily.
4. Turn the water valve open and let the tank fill.  Flush the toilet a few times.

Repeat this process whenever you see the unsightly ring reappear.