How come my wife sim has brown hair, my husband sim has black hair, and their child has blonde hair? How did this happen? Aren't the black and brown shades dominant?

Ok, this is how sim genetics work.
Let’s start with eye color. The recessive colors are gray, light blue and green. The dominant colors are brown and dark blue. The super dominant is any custom eye color that you have made or downloaded. This being the case, the dominant colors, brown and dark blue will overpower the recessive colors, green, light blue and gray. Any custom color will overpower the dominant and recessive colors. So, if you have two sims that you have just created (sims with no history of parents) it’s easy. Mom has brown eyes, Dad has gray: all children will have brown. Mom has green, Dad has light blue: children could have either one. Mom has brown, Dad has those pretty purple ones you downloaded: all children will have purple.

It gets more involved when the sim carries genes from a parent, like the children of the sims you created. When they grow up and have children they pass on alleles. When mom and dad have little Johnny they each give him an eye color. One he takes, and one he carries with him in his DNA. So now it is possible for two brown eyed sims to have a green eyed baby. Say Mom has brown eyes and carries her dad’s green, and Dad has brown eyes and carries his dad’s light blue: baby could have brown of course, and then with the parents alleles of green and light blue he could have either of those too. It’s like a roll of the dice, and whichever two colors come up, the more dominant is what the baby will have, if both are not dominant, or if both ARE dominant, it’s another roll of the dice. Mom has green carries gray dad has green carries brown.
1st child roll of dice : for mom- gray for dad brown = baby gets brown
2nd child roll of dice: for mom-green for dad green = baby gets green
3rd child roll of dice: for mom-gray for dad green = both colors are recessive so now we roll the dice again to see which one baby gets, and of course, baby also takes an eye color to carry.
It works the same with custom content, only custom content is more dominant than the dominant colors.

Hair color works the same way. Recessive colors are blonde and red, dominant colors are brown and black, and Custom content is super dominant. If mom has custom content hair and passes that custom content onto her daughter her daughter will have the same hair, but if mom passes it on to her son, the gene will revert to the base hair color from which the custom content was created.

Skin Tones:
Child can have skin tone of one parent or anywhere in between. For instance, let’s call the skin tones S1 S2 S3 and S4, S1 being lightest, and S4 being darkest. Skin tones are not dominant or recessive and are totally a roll of the dice. For example if mom has S1 and dad has S4, baby can have any of the skin tones from S1 to S4. If mom has S1 and dad has S3, baby can have any from S1 to S3. NOW, if there is parent history, it works the same as the eyes and hair, child takes one skin tone from parent, and carries one.

This means that lightest skin tone sims could go for generations of having lightest skin toned children, and then all of a sudden, the darkest skin tone allele could pop up and two lightest skin toned parents can have a darkest skin toned baby, and vise versa. Try mating Hermea in Veronaville with a lightest skin toned man and have a few children. :o)