I installed a sprinkler timer on a silcock outside the house, to
irrigate some new grass seed I planted. When the timer turns the water
on, there is a brief episode of water hammer before the water starts
to flow.

Ever since I put the timer on the spigot, there's been black water in
the bathroom fixtures. At first I thought it was a disintegrating
rubber washer in the toilet. But the black water has come out of the
sink and shower too.

The black water goes away after a few seconds.

I imagine I need to flush my hot water heater. But does anyone know
why this might be happening?

We are on city water. The timer had previously been installed on
another outdoor spigot, and this had never happened.

What to do?