I think that the problem is the camera exposure setting for the ambient light is way underexposed. The D-TTL setting on the flash attempts to balance the ambient light and the flash resulting in overall underexposure. I had a similar problem and got the following response from Thom Hogan:
"What happens with the D100's internal flash? Do you get the same results? If not, send your SB-28DX back to Nikon for adjustment.
But your photos show some misunderstanding of how the Nikon flash system works. First, you're setting a background (ambient) underexposure. Actually a considerable one. In balanced fill-flash modes that almost certainly will result in lower flash output than desired. Remember, in balanced fill-flash you asked the camera to BALANCE the ambient and flash. So if the background is underexposed, what do you expect the camera to do? That's right, underexpose the subject lit by flash."
I'm going to try a few photos using standard TTL since this flash mode does not try to balance the ambient light with the light on the subject.