What blood type will a baby have if the parents both have the same blood type?
The two AB parents could have any of an A, B or AB, but not O.
The two A parents could have either A or O children, but not B or AB.
The two B parents could have either B or O children, but not A or AB.
The two O parents could only have O children; they cannot have have A, B or AB.
If both parents have rhesus-negative blood, so will their children. If both have positive, the child might be either.