Tiny black bits left in the bowl/sink after running hot water. What are the?

Had Greenstar Heatslave 18/25 oil fired condensing combi boiler (Worcester Bosch) installed 4 months ago. The hot water fairly frequently turns to dirty water then clear then dirty. Also for the majority of the time there are tiny black bits left in the bottom of the bowl/sink.
I'm told they will go in time but what are they and will they just stop appearing & will the water eventually settle?
Thanks