Photographing Black It has to do with exposure. Here's a good link
Link The lighter the color your camera focuses on the less exposure it thinks it needs. The darker the color your camera focuses on the more exposure it thinks it needs. So if you point your camera at a black object, it will allow more light in to give the picture more exposure. That's why your black dress looks gray. The camera added too much exposure.
The way I photograph black clothing is to turn off my flash. Put the item against a white background. Aim my camera at the edge of the item so the white is more in the frame than the black, push the shutter button down until it focuses, now move the camera to center my item and push the shutter button all the way to take the picture.
This method takes some trial and error, but once you get it figured out for your lighting situation, it works great. And like others above said, editing software will take care of more adjusting.