Join the club! The last 6 months i found that i had the same problem and i have a culligan softner. The smell is sulfer or magnesium which leaves stuff black. We had a deironizer and sulfer conditioner put in on line before water conditioner and now they tell me the W.C. is pluged and making the water back to worse cause the softner is after the unit they put in so i’m back in the same boat. But i bought a water heater thinking it was that but found the W.C. was plugged full of magnesium and sulfer which smelled like sewer water right. hope i helped you.