Jam
I can only go to the late 1800s. in response to your question.(Thank goodness for that I hear them cry)
The "non white" ( for want of another word) population in Liverpool traditionally lived in a certain area near the river/docks.
Not just black people, but a big Chinese community too. (The Jewish community a little further out, but tightly knit again.)
Lots of reasons for that population to live where they did - work, a sense of identity, lack of access to decent housing, (no Blacks, no dogs, no Irish) racism from authorities, and so on.
Into this, Liverpool also was a City with a religious sectarian history too Irish, Protestantism and Catholicism.
Consequently, people tended to stick to their communities, and tended to marry from within them, or from around them. (I'm talking late 1800s to 1950s, things began to change after that, but still that area remains the most diverse.
The Windrush generation came post War, and settled, so we of course mixed socially, as younger people, but no direct family Windrush relations afaik. We socialised in places like the notorious Somali club in Liverpool. My husband's mother (black) and sister, very religious, never travelled out of the City in their lives other than than the local seasides. His Dad (black) worked on the ships as unskilled labour.
My own famiy line is mix of black Liverpool born/ white Irish/ Chinese prior to 1948. All poor, and many associated health problems, For example, my black paternal grandfather had three cleaning jobs. One job was brushing up loose asbestos on ships at the docks. He died in 1962 of stomach cancer. Tough lives. I found him on the Liverpool census for 1901, he was born in Liverpool.
I was brought up by white Irish relatives after some time in care, ( I won't expand on that bit sorry) but am dark skinned, as is DH. So, some might call me "mixed". It's not a term I am personally comfortable with, but understand why some may choose it.
I've no living family, apart from an "assumed white" cousin in the USA. who emigrated from Liverpool in the early sixties.
Husband black/ white Irish/Italian heritage (no living family) is from the same City, we knew each other from school. We made our own way, and did ok. Generally, our life is good.
DD's husband is white. One child is a lighter skin tone, than the other.
I imagine that if they marry white partners, their children may be "assumed white."
Apols for any typos
I 'm bound to have missed something.