It depends on the genotype of the Rh+ parent. If that person is heterozygous, then yes there is a 50% chance of an Rh- baby.
Father's Group (negative)
Mother's Group (positive heterozygous)
Rh +Rh -Rh -
Rh +, Rh -Rh -, Rh -Rh -Rh +, Rh -Rh - Rh -
http://www.bloodbook.com/inherited.html
If the positive parent is homozygous, the baby will be positive:
Father's Group (negative)
Mother's Group (positive homozygous)
Rh +Rh +
Rh -
Rh +, Rh -Rh +, Rh -Rh -Rh +, Rh -Rh + Rh -