Within the past few days there has been black silt in the toilet bowl. It does coat everywhere under the water and does flush away, but comes back. Our well is 475 deep and new as of 2 years ago. Not noticing silt anywhere else but in the toilet. Only recent change is the addition (which I do yearly) of the enzyme stuff to keep the tank clean. Dogs do not have access and there is the normal rusty/sludgy color in the tank. Toilet doesn't have quite the 'flush power' it used to, though. Oh - house is 30 years old and system has never needed to be pumped/cleaned. We are not home much so the toilet is not used as much as it might be if there were a large family. I

If the silt is only noticed in the toilet bowl, I would suspect a deteriorating flapper.

Take your hands and feel the flapper. If they come away black, replace the flapper. Check the water level in the tank ( water pressure) it should be 1/2 inch below the top of the white over flow tube. Also, you might check the small jet hole in the toilet to see if it completely clear. If it isn't scrape and remove the build up.
Good luck, Tom