I am not sure that I completely understand and this may not be as helpful as you need it to be; however, as far as blood typing goes I can help. O is a very common blood type and determined by several factors, alleles, that determine blood type by two primary alleles, a dominant one and recessive (hidden) one.

So for a "B-" Blood Type mother to have an O Blood Type child, she would have to be "BO - B dominant, O dominant " or "Bo- B dominant, o recessive" Type. The father could either be in the case of an Type O child the father would have to be:

-Type A: (AO) or (Ao) or
-Type B: (BO) or (Bo) or
-Type O: (OO) or (Oo),

then a Type O is probable. With 2 parents Type B, whether BO or Bo for either of them, there is a 25% chance the child will be Type O, not the rate you quoted earlier.

The only way to know for sure is to go for testing because there is no way to know those recessive alleles without testing.

I am sorry I cannot narrow it down for you further in your quest for answers. I wish you well in finding what you seek and peace for you and your family.