OK so I found an old thread here forwarded from a Yahoo Answers question.

So, after a customer master shower install, we haven't used shower #2 in a while. This last stretch was about a month. Turned on the hot water today & noticed occasional spurts of black stuff. My jaw dropped. Lasted for about 30 seconds or so and spurted about every 3 or 4 seconds.

14 yr old house
4 yr old GE 12yr heater from HD
Mixet valve

In regards to the black stuff, it appeared to be water soluble as it appeared to dilute in the water and quickly disappeared. No stains, or settling chunks I could hang onto. Wife saw it too, so I definitely wasn't seeing things.

Any insight appreciated.