According to the Canadian Children's Rights Council, if a mother has A negative blood and a father has O negative blood, the baby has either A negative or O negative blood. Type A is more likely, because it is dominant over type O.
The University of Arizona explains that each parent contributes one version, or allele, for blood type. Since O is recessive, the mother must have type OO blood. The father has either type AA or type AO. If he has AA, the baby's blood with have to be A, because A always dominates over O. However, if he has type AO blood, there's a 50-50 chance that the baby receives an O allele from him. In this case, since the baby only gets an O allele from the mom, the child has type OO blood.
The RH negative factor is recessive to RH positive, the Canadian Children's Rights Council states. However, since both parents are negative, the baby is, as well.