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Yes, even if two parents have dark hair, they can have a child with blond hair. There are two factors, which are known as heterozygous and homozygous.
Yes, if both parents are heterozygous (BB). The dominant color is black (B), while the recessive color is blonde (b). Because there is no dominant gene to repress the blonde characteristic, the trait is now manifested if the child receives the “b” allele from both parents. The genotype of the child is bb. If both parents are homozygous dominant, the child cannot have blonde hair (BB).
Homozygous describes the genetic condition or the genetic state where an individual has inherited the same DNA sequence for a particular gene from both their biological mother and their biological father. It’s often used in the context of disease. We talk about a situation where an individual has inherited a mutant allele or an error in DNA sequence from their mother and they have inherited the identical mutant allele from their father. We would then say that individual is homozygous for that mutation. They have two identical copies of the deleterious version of that gene and, as a result, they are then going to be predisposed to the genetic condition the gene codes for.
Heterozygous is a state of having inherited different forms of a particular gene from each one of your biological parents. Now, by different forms we generally mean that there are different portions of the gene where the sequence is different. They may be inconsequential portions of the gene, or they may in fact be pretty important portions of the gene. That doesn’t really matter for our discussion today. The word “heterozygous” simply means that your biological mother and your biological father, when they contributed their copies of a particular gene to you, they did so in a way so that the DNA sequence is slightly different. It can be different at one point in the gene, or it can be different at dozens and dozens of different points in the gene. Now, a heterozygous genotype stands in contrast to a homozygous genotype. And in the case of a homozygous genotype, we’re talking about a case where we’ve gotten identical forms of a particular gene from each biological parent. That is, if we were to read along the DNA sequence that mom gave you and the DNA sequence that dad gave you, we would find absolutely, positively no differences in that gene or in the region of the gene that we’re concerned about. “Heterozygous” meaning different, “homozygous” meaning the same.
Source: https://www.genome.gov/genetics-glossary/heterozygous
