The overall population of women with a negative blood group have about a 63% chance of having a baby with a positive blood group (therefore a 37% chance of having a baby with a negative blood group).
The chances of having a baby with a specific blood group will depend on the blood group of the father of the child. If the father has a negative blood group (like the mother) then all the children from those parents will always have a negative blood group.
If the father of the baby has a positive blood group then there may be a chance that the baby will have a negative blood group, depending on the type of positive blood group the father has.
Although the concept is not easy to explain (or understand), it helps to know that there can be 2 different types of 'positive' blood groups, being heterozygous (60% of men) or homozygous (about 40% of men).
'Heterozygous' means the man's blood group is a mixture of the 'positive D' and 'negative d' component. The positive 'D' is stronger or more dominant than the negative 'd' (thus overriding it), causing his blood to test as being a 'positive' blood group (even though he carries the less dominant negative 'd'). Therefore even though the man's blood is positive, the less dominant 'negative d' can be passed on to the baby in 50% of cases, enabling the baby to have a 50% chance of having a negative blood group.
'Homozygous' means the man's blood group only has the 'positive D' (and no 'negative d'). Men with this type of positive blood group will always father babies with a positive blood group because they do not carry the 'negative d' component.