Making this a quick answer or suggestion.
Change the water flex hoses on your water heater cold and hot. The gaskets are black and are now leaking into the system.
Shut-off cold coming into your water heater, open the hot side of a water fixture to drain your hot line, and just unscrew one your hoses from the water heater and touch the gasket and you'll most likely see that same blank ink that you see and goes away.
I spent a lot of time at a service call on a home to finally figure this out and they had a very complex and after market products on their system. 3 or 4hours later it was just the flex hoses.
the black water comes from deposits in your pipes that build up over time, occasionally these will beak loose and cause the discoloration. they aren't hazardous just let the water run until it clears up. the only sure fire fix is to replace the pipes. and that's really not cost effective. just let the water clear up when it happens. so many factors can cause this the temp outside the water pressure someone working nearby shaking the ground or pipes. that's why its so random. It wouldn't hurt for you to drain the hot water tank as the deposits are likely settled in it as well. it's easy to do.
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