У членов семьи не blue card, а FR к держателю blue card .

The family members of the holder of the blue card can apply for a family reunification permit or continue having the current FR permit. They can apply simultaneously with the sponsor. No need to wait for the permit of the sponsor to be approved and then to proceed with the family application.
 
Please note that family members need visa to travel to Schengen area. The extension is only for the holder of the blue card. Blue card is only for the employees, not the family members.