This is my opinion, and it may be no one else's. There is no such thing as "passing for white." If you are "white presenting," then you're white. To consider someone as anything other than white in that circumstance, is to subscribe to the "one-drop" rule. If you're 98% white and 2% black, you're not black. You're white.
There may be those who celebrate their black origins or those who hide them, but if those two people look exactly the same, then one is not "white presenting" and the other "passing;" they're both white. Or, they're white until the Brits decide to take that two percent or five percent, or whatever, and decide that you're not.
If you look at photos of the Duchess as a teenager, then you would certainly know that she has some African in her. Her hair is frizzy, and her nose has a little characteristic flatness. Like you, however, I'm not here to talk about how much of her is what. I'm here to agree that there are some who, for whatever reason, believe that any African-ness you possess must be used against you.