First off, I am not a geneticist. DNA, genes, and all that jazz are not my specialty at all. I, in fact, harbor a deep dislike of biology. This is why I went into aerospace engineering. However, that being said, I did take biology in high school and know the basics of dominant and recessive genes.
It seems to me that many modern authors have forgotten their ninth grade biology.
I have gotten this impression from several books I have read recently, books where I have been jarred by a child simply having the wrong hair color, eye color, or skin tone. (I say skin tone instead of skin color because it is much much much more complex than "black", "white", "brown" color words that people use. And not just politically correct complex. I mean like genetically complex).
So let me remind you of Punnett Squares.
Punnett Squares are this simplifying tool people use to predict what trait a child will have if their parents have certain traits. For example, the simplified eye color Punnet Square. In this square capital B means brown and lowercase b means blue. Got it? Ok. So let's say the mom has brown eyes with a recessive blue gene (meaning one of their parents or grandparents had blue eyes). The mom then has the gene 'Bb'. But the dad has brown eyes with no recessive gene trait. What is the probability that the child will have blue eyes? Let's look at our Punnett Square.
Eyes are actually more complicated than the 2x2 square indicates. I found this blog post that describes the more complicated square that takes into account that people can have green and hazel eyes. The writer does show it's possible (but extremely rare) for parents with blue eyes to have a brown eyed child. But the matrix just as easily shows that blue eyes are the most recessive of eye types, and therefore, theoretically, the rarest.
So why did I go to the trouble of explaining all of that? Because if both of your character's parents have blonde hair it is highly highly highly highly unlikely (if not impossible) for the child to have black hair. If both of your character's parents have brown eyes, it's most likely that your child should have brown eyes. Don't just make these things up. Don't just think "Hey wouldn't it be cool if the mom was blonde hair/blue eyed, the dad was red hair/brown eyed, and the kid was black hair/green eyed."
It's not cool. It's wrong. And your readers will know. Especially if you write YA, because guess what--teenagers have to take biology.
So do your research and think about the genetics. You don't have to get complicated, just write out the basic Punnett Square. Give easily overlooked stuff like genetics an actual thought and your readers will appreciate it.