Can Two Positive Parents Produce A Negative Blood Type Baby?

Short answer, yes.

Long answer;
both parents would have to be +/- since everyone has two alleles to determine pos or neg. With +/- blood you show as positive because it is dominant over the negative. If each parents donates a - then the baby will be -/- resulting in a negative blood type.
Basically, there is a 25% chance that the baby will be negative. Here are the possible donation scenarios:
Mom dad
+ / + (positive)
+ / - (positive)
- / + (positive)
- / - (negative)

If the Father is A+ the mother is B+ can they produce an O- baby?

Two positive parents can produce a neg. Child but only if they are both carriers for the neg allele and both pass it on  as neg. Is recessive.