When you say there are 12 dark pixels attached to its ADC, does that mean your camera sends 12 dark pixels at the beginning or end of every line? If so, you should be able to acquire those pixels and read their values. If they always appear in the same location, I would use the IMAQ GetPixelValue.vi to return the pixel value. This VI takes the pixel coordinates as an input and outputs the value of that pixel.
Please let me know if this won't work for you.
Kyle V
Applications Engineer
National Instruments