The presence of a blond-haired offspring, which is a recessive phenotype, indicates that both parents must carry at least one allele for the recessive trait. Since black hair is dominant, the possible genotypes for black-haired parents could be homozygous dominant (BB) or heterozygous (Bb).
If both parents were homozygous for the trait (BB), they could only produce offspring with black hair. Therefore, both parents cannot be homozygous for the trait.
If one parent is homozygous (BB), the other must be carrying the recessive trait (the only possible genotype being Bb) to produce a blond offspring. But that would mean only one parent could be homozygous.
The only scenario where both parents can contribute to a recessive phenotype (blond hair) is if they are both heterozygous (Bb). This would allow for the potential of producing offspring with the recessive phenotype when two carriers are crossed.
Therefore, the true statement about the parents' genotypes is:
• Both parents are heterozygous for the trait.