Although this navy blue color was used specifically for depicting the Union Flag on maritime flags on the basis of durability , it soon became standard on Union Flags , both on land and at sea . When incorporated as part of the Union Flag during the 17th century , the dark blue applied to Union Flags destined for maritime use was possibly selected on the basis of the durability of darker dyes , with this dark blue shade eventually becoming standard on Union Flags both at sea and on land .