let say her parent's blood type is OO which will come out as O, and BO which will come out as B
according to mendell's law, the probability of their child's blood type will be:
OB = B
OO = O
OB = B
OO = O

then there's dominant and recessive gene where:
dominant+dominant = dominant
dominant+recessive = dominant
recessive+recessive = recessive

rhesus(+ve blood) is dominant, while no rhesus(-ve blood) is recessive

now let's say her parent's blood type is O+O-, which will come out as O+, and O+B-, which will come out as B+
the probability of their child's bloodtype is:
O+O+ = O+
O+B- = B+
O-O+ = O+
O-B- = B-

don't quote me for this, as this is what i remembered from what i learned for spm 6 years ago.

O- is not that rare actually
common to rare:
O+
A+
B+
O-
A-
AB+
B-
AB-

but since in an emergency, the doctors wouldn't have enough time to check the patient's blood type so they would just give out O- first, that's why at blood bank O- would be considered rare.