sounds like manganise or the like to me. see how much of this stuff is in the drain valve of the heater. there is no carbon filter inside the heater, whatever it is, it’s most likely coming through your water lines. wait a second, does it only do this at the tub? you could have a black iron nipple on the tub spout. its also possible that the water heater is rusting bad, but i dont think so. ask any water treatment guy in your town if theres strange black stuff in your water. jack