The thing is... In many ways ethnicity is a relatively fluid concept.
My DH is 75% black (carribean/West African mix) he is quite dark skinned however our son is very light. He's only a toddler so difficult to know how his skin tone will evolve but people won't consider him "black"
I'm not sure what his relationship will be with black identity. It's an enormous part of DHs life, not only with how day to day experiences but he runs a number of diversity projects at a very large company. But we don't know how our son will feel.
What I'm trying to say is there is no universal experience and someone who is very light skinned mixed race can choose to strongly identify with their black ancestry or not. We accept this when it comes to ethnicity but there is zero doubt in anyone's mind that you can not change ethnic groups. Yet we have the belief that gender is so fluid. I don't understand why this is.
A bit long winded but hope there's some coherent thoughts there!