Welcome to the forum Stangbanggang! It's always nice to see new members around.
What about more "exotic" skins? Are they available somewhere?
I can absolutely confirm this as I've seen tons of custom skins in use on other sites, but I'm afraid I don't use them myself so I don't know where to get reliable, good quality ones. Unlike in The Sims 2, which provided an offical Body Shop that you could make your own skins with, Sims 3 skins have to be make with third-party programs that may or may not be any good. You might want to check around a forum that's involved with the CC community.
No idea where the black gene came from! The blue dad I'd created specifically for this family, and the white mum was adopted as a child, maybe she had mixed race parents and just ended up quite light skinned? Or maybe she was actually black but albino, although I doubt The Sims includes albinoism.
I actually have a theory about this. My idea is that skin "colour" (the particular scale of light to dark you picked from the six options in the menu) and skin "shade" (how far along the scale the skin colour is set) are independently inherited traits. So, Sims can get their skin "colour" from one parent and their skin "shade" from the other. By the sound of it, the baby of your family got his normal skin "colour" from his mother and his dark skin "shade" from his father.
I'm pretty sure adopted childen don't have any special hidden genes, which is a shame.