Around my area, I get the same thing in the newer houses, mostly in basement remodels. But another thing we get is.....mill dust, or coal dust from the many years that this area was a top steel producer. They had very little pollution control back then (up through the 70's), and that dust got every where. There are still plenty of houses that I clean that still retain that dust under the baseboards. Bump 'em and they keep spilling out the crap. You just gotta turn the wand sideways, and finish up along the wall. Or cuff it.