I can relate to this woman in a way, but I'd say that being trans-black is impossible.
When I was a young kid, I thought of myself as an African American because I had a black parent. As I grew up, I began to realize that nobody else saw me as part of the black community (based purely on my own physical appearance), and my self-identity didn't match up with society's imposed identity. My reaction was to accept society's imposed identity, and I more-or-less rejected my early self-image as a person of black American heritage.
I will never be a black person ever, no matter how entrenched I was in black culture at my home or amongst my extended family, or even because I can trace ancestry to west African. Rachel may find home in black culture, but culture is meaningless. Race is based on phenotypes and phenotypes alone. Unlike transgender, the underlying phsiologically structure of race is too complicated and prominent to be convincingly altered by plastic surgery.
Recently, I tried to join a black professional network to help people reach their goals like I have. The shifting and questing eyes, the nervous greetings and untrusting gestures really made me feel uncomfortable and unwelcomed. It is clear that I am not part of that tribe, but that is okay. I have to accept who I am, and Rachel must accept who she is as a white women.
Race is not a social contract. It is based on biology. It is clear that the social construct theory is being challenged by Rachel.