Answer: Hard to say, assumable 50/50, child will still carry recessive gene.
Explanation: Dark haired genes are dominant meaning if your child gets one piece of a gene(s) from parent Bk that is(are) dominant, they will likely have or develop dark hair as the recessive gene(s) are overpowered. This is why some children are born with blonde or light hair that develops into dark brown/black as they age.
Theoretically, Parent Bk would have to have a part of a blonde recessive gene(there are multiple genes but even one is enough) to give to a child in order for them to have blonde hair. Or, the gene(s) they receive from parent Bk malfunctions and the recessive gene(s) from parent Bd is(are) the only one(s) expressed making the kid blonde. If the father has black hair and nobody in his family or ancestry has blonde hair, then there is still a chance that the child could get blonde hair depending on the mother. Because we don’t know for sure what alleles parent Bk has, you could end up with children who all have dark hair(not explicitly black but dark) or 50/50 dark/light
