If both parents have black hair, which is a dominant phenotype, and one of their offspring has blond hair (a recessive phenotype), it indicates that the parents must each carry one allele for the recessive trait (blond hair) in addition to their dominant alleles for black hair.
In genetic terms, we can denote the dominant allele for black hair as "B" and the recessive allele for blond hair as "b". The genotype for black-haired parents could therefore be either homozygous dominant (BB) or heterozygous (Bb). However, since they have produced an offspring with a recessive phenotype, it is clear that both parents must be heterozygous (Bb).
Thus, a true statement about the parents' genotypes would be: Both parents must be heterozygous for the hair color trait (Bb) to have an offspring with the recessive phenotype (blond hair).