Definately build up of brake dust. you can either take the wheels off, and clean the brake disk with a cloth etc, or try to blow the dust off using a forecourt air line. Either way try not to breathe in too much of the dust - it will be asbestos free, but still it is dust.
Alternatively, find an open area (car parks are best). Stick the car in reverse, drive backwards and apply the brake GENTLY with your left foot, remember GENTLY. Run the car like this for a while and this should clear the dust from between the pads and disk