Heterozygous A is AO
Heterozygous B is BO
Possibilities for offspring are:
AO 25%
BO 25%
AB 25%
OO 25%
So there is a 25% chance the offspring will be AB and a 25% chance the offspring will be OO. That adds up to a 50% chance that the baby will end up with a different ABO blood type as either parent.