The RH factor makes your blood type rare. Only about one in 15 people are O-. You should be thanking your stars you are not AB negative. It is the rarest blood type. Only one in 167 people are AB negative. I think they can store rare blood types for up to 10 years. Normally blood is only stored for 6 weeks and then it is disposed of. In my youth, a friend of mine was an entomology student. He used to get expired blood from blood banks and feed it to his (i.e. the university's) insects.
O- is also the universal donor. O+ is popular for a similar reason. I and my daughter are O+, my wife and my son are O- which suggests that I probably carried the rhesus negative recessive gene.
I am an AB+ blood type and my children's father was an A, when I learned that my son was B- I thought how in the hell did this happen. For years I thought that if both parents were positive that their children would also be positive but I have living proof that this is not true.
I am also AB+... But, my dad was O-, and my mom was AB-... Figure that one out. Two negatives created a positive. I wonder what blood type our old mailman was? Hmmm.
My recollection is that rhesus factor is a classic dominant/recessive trait. If both you and your husband both have family members that are rhesus negative, then you will carry the rhesus negative genes as a recessive trait, consequently, there is a 25% chance that one of your kids will be Rh-.