Please see this web site http://www.ianr.unl.edu/PUBS/water/g1280.htm Iron and manganese are chemically similar and cause similar problems. Iron is the most frequent of the two contaminants in water supplies it will cause the water to have a reddish tint. Manganese is typically found in iron-bearing water and will appear as dark or black tint in running water as you described. These problems are easily dealt with. Your first coarse of action would be to test your water to find out the concentrations of this contamination. You will want to confirm that this is your problem with the water test data. I would suggest calling a local water treatment professional to aid in your decision. Low to moderate levels of dissolved iron, at less than 5 mg/l concentrations, usually can be removed using an ion exchange water softener. Be sure to check the manufacturer’s maximum iron removal level recommendations before you purchase a unit. Capacities for treating dissolved iron typically can range from 1 to 5 mg/l. Oxidized iron or levels of dissolved iron exceeding the manufacturer’s recommendations will cause a softener to become plugged.
High levels of dissolved or oxidized iron and manganese greater than 10 mg/l can be treated by chemical oxidation, using an oxidizing chemical such as chlorine, followed by a sand trap filter to remove the precipitated material. Iron or manganese also can be oxidized from the dissolved to solid form by adding potassium permanganate or hydrogen peroxide to untreated water. This treatment is particularly valuable when iron is combined with organic matter or when iron bacteria is present. I strongly recommend having a professional use the later of the two treatments. Potassium permanganate is dangerous to have around the house and should only be used by water treatment professionals.
Respectfully David F. Walling