What are the possible blood types of a child that had parents that were both heterozygous for "B" blood.

Step 1: Determine what blood type the parents are.
The parents are heterozygous for “B” blood meaning that the parents have 2 different alleles “B” & “O” for their blood type of “BO”. (Fyi, If parents were homozygous, their alleles would be the same or “BB”.)
2. Determine the possible blood types of their child.
Remember that children get 1 allele from each of their parents for a trait, so..,

Parents are: BO & BO

Children could be: BB, BO, OB, or OO = Type B, B, B, O, respectively.

So, a child could be Type B or Type O.