BJCP Category for a Black Witbier

Well, technically, you can only lose up to 3 points for appearance. However, as people "drink with their eyes", I think you'd struggle to do well amongst other witbiers in the category, assuming it's not disqualified up front for being entered out of style.

I'd personally enter it as a specialty beer and give enough info that hopefully the judges would choose to judge it in an appropriate order. If you declare witbier as being the base beer and it matches the other stylistic components of a witbier, then you should have a good chance at doing well with it. Witbiers have a very characteristic flavour profile, so I think you'd run in to problems in any other category, with the possible exception of 21A (Spice, Herb, or Vegetable Beer). Even in that category, there is a chance you could be marked down for being out of style with the base beer.

If it is an option for the competition, you could enter it multiple categories; once as a witbier and once as a specialty beer and see how you go. Another useful thing to do is judge it yourself under the different categories and see how it compares to the descriptions.

Category 23 is pretty much it. There isn't another category where the flavors of witte would fit with that dark color. People who are making CDA are currently stuck with Cat 23. The secret to cat 23 is making a beer that is completely flawless. So that the "greatness" of it can shine through despite it not having a defined style. Second to that is that if you are doing something different that difference must be the showcase.

In your case, that black witte better be black, not brown or tan.