That's probably most plausible explanation, though I suspect same as vast majority of fantasy and fiction writers they just don't have many examples to observe and portray from real life.You know what? Weird credit where it's due. It doesn't seem like mom was getting blamed for having a black-haired child at all despite the ostracization of them.
Considering they're cozily sipping wine together, their relationship seems fine.
In fact, why doesn't Yumellia have any siblings? Are they afraid of producing more black-haired children?