What was a black substance on the surface of cooked brown rice?
If this is the grey foam/scum that rises during the cooking, and eventually settled onto the surface and the pot itself, then it's just starch that gets released during the cooking process.  While not aesthetically pleasing, it doesn't change the flavor or texture of the rice, and is fine to eat.

Also, some people note this when their water source is hard water, so it might be the excess calcium/limescale from the water, itself, just like what accumulates on your faucet or in your coffee maker, but mixed with some of the previously mentioned starches from the rice, so it looks different.

(I'll try to find some authoritative links to confirm this, and post those as an edit when I have more time after work).