Answer:
The hair color of all offspring will be black.
Explanation:
As you can see in the punnet square below, if the parents have the genotypes "BB" and "bb", all offspring will have the genotype "Bb".
The "B" allele is dominant and represents black hair. In contrast, the "b" allele is recessive and represents white hair. The dominant alleles express the characteristic they represent even if they pair with the recessive allele (which is only expressed if it pairs with another recessive allele). In this case, since the offspring receives the "Bb" alleles, the dominant allele was the expressed allele, so the entire offspring has black hair.
