All types of combinations are possible actually.
If the father is blood type A, this means that his genotype could be AA or AO.
If the mother is blood type B, this means that her genotype could be BB or BO.
If the mother is BO and the father AO then you can get these possibilities:
AB AO BO or OO. This means that all different blood types can be obtained.
But is the mother is say BB and the dad's an AO still then you'll get:
AB or BO.
And if the mum is BB and the dad is an AA, then all the kids'll be ABs.
(you can draw a Punnet square to get the possibilities).
http://en.wikipedia.org/wiki/Punnet_squareAs for whether the kid'll be Rh+ or Rh-, all possibilities are also possible.
Since both parents are Rh positive, their genotype can be:
Rr, rR or RR.
If the mum and dad are both Rr then you'll get,
RR, Rr, Rr, and rr. This means that there is a 25% chance that the kid'll have an rr genotype resulting in Rh-.
But the likelihood of a child possessing a genotype for Rh- is quite low and depends much on ethnicity.
go here for more info:
http://en.wikipedia.org/wiki/Rhesus_factorSo when you say that the fathers blood is A+ and the mum's B+, all possiblities are equally likely. We need more detailed information to determine the exact genotype of the mum and dad (whether homozygous or heterozygous). This may be obtained by looking your mum and dad's brothers and sisters and their parents and a pedigree chart can be made.
Hope this managed to help you even if it's in a teensy weensy way. have fun with genetics! its really cool.