'04 Focus tdci, black grime on wheels (not road grime)

Have a 2004 Ford Focus tdci. I have noticed on the front wheels a coat of black stuff that is not normal road grime. We bought the car at a used car dealership. We noticed at that time that there was rust on all the brake discs. We were told that this was normal because the car had not been ran since it had rained the previous week. When we test drove it, the rust came off and they discs looked fine. However, when we bought the car it was negotiated to have them replace the brake pads and discs. Now, the two front wheels have this coating of dirty black stuff after a month of the brake job. I know this isn't normal road grime. What could be causing this? Thanks for your time.