I would imagine the sensor would give some kind of error. It could also be your shutter has failed and isn't opening. Either way, not good. Check the latter by putting the camera into manual focus, removing the lens and taking a picture while looking in. Longer exposures would help you see this better.
Try one more thing. Take a longer exposure, say about 3-4 seconds. It could also be that your lens aperture closing down all the way and the exposure is just too short to show it on the screen. This could be a dead camera, dead lens, or just a dodgy contact. Again look into the lens to see if this is the problem.
Basically a Dark frame could be caused by a few things, and Nikon techs are idiots, so the more you could narrow down the problem for them the better.