There are three main reasons this might take place.
Black water coming out of bathroom sink faucet.
The reason for black water running out the hot faucet is.
Sometimes the water softener can break down and the resin beads can get out and into the water system.
Alternatively the sediment accumulated in the water heater could turn water black.
The old rubber in the flex hose started to deteriorate and rubber particles got into the water.
If you saunter into the bathroom and turn on the faucet to find a burst of black water you likely want answers right away.
While this mineral is often present in the water supply when it mixes with oxygen it can turn black.
What causes black water coming out of the faucet.
The corrosion that builds up is usually black in color.
Replace the stopper and lock it back into place.
Left unchecked the corrosion can continue until the galvanized pipe completely dissolves.
Ergo we have our second source of black flakes.
Take the stopper outside and clean it thoroughly putting the nasty mess in the trash.
A few spurts would occur usually shortly after the faucet was turned on however i also seen it happen after running the hot water for a couple minutes.
Manganese could also be the external cause of black water.
Natural resin beads from your water softener are often black in color like charcoal.
Clean the sink stopper from which the black gunk is coming.