I was thinking about that a couple weeks back. I would assume at this point that it will be black only. Seems weird to me, however it makes sense if apple is considering this phone to be something unique, compared to the 7S and 7S+ that would be the normal iPhone's released this fall.
Also, when considering the frame is going to be black steel similar to the black Apple Watch stainless steel, then it makes sense a lot of detail is being put into it.
It would mean that similar detail would need to be put into a white version, maybe ceramic-like look to the frame (or regular silver steel), and also that we would have seen some sort of leak or at least information regarding a white version, but we have not.