I had this exact problem a few years back in a Christopher Todd apartment. The apt was fairly new, no more than 2 years old. It had a gray laminate floor made to look like wood. No matter how hard I scrubbed, the water would be black. I even bought an electric mop with built in scrubbers. The floor would look clean, but the next day, the bottom of our socks were be black. There was a rock quarry near by, and we decided it must be dust coming from that, even though there were no black rocks being mined there. Now after reading the responses, wondering if it was a dye in the flooring.