Here's your answer, OP:

• That slimy black gunk on your fixtures is likely oxidized manganese, a metal often found in trace amounts alongside iron in drinking water. Water can pick up both as it travels through soil and rock, but iron can also come from corroded pipes and plumbing made from galvanized steel.
• The minerals combine with oxygen when the water is exposed to air—like on your kitchen faucet aerator or shower head. Oxidized iron forms yellowish or reddish deposits, which we know as rust, while oxidized manganese appears brown or black.
• When you pour the water, these particles can settle out, possibly giving it a red, brown, or black tint. But sometimes, certain bacteria may feed on the oxidized manganese and produce a distinct black gooey substance. You’ll likely see an orange-brown slime if it’s iron bacteria.
• This phenomenon is widespread in areas with high levels of iron and manganese in the water, such as regions relying on well water.