Cause of black specks in cold water from kitchen mixer tap?

I've started to see black specks coming out of the cold water mixer tap in the kitchen, this happened yesterday afternoon. I've run the hot water from the mixer tap and there are no black specks, they only appear when the cold water is run. I've also checked other taps in the house, all mixer taps and they are OK when running cold and hot. I've also checked the outside tap (in the garden) which I believe comes from the same supply in the kitchen and that was also fine. I used a spoon to pick some of the specks out to squash between my fingers to see if it was rubbery, but it disintegrated into an ink like substance. Does anyone know what this might be or what might be causing it and ultimately how to fix it?