I would like to first draw your attention here if you haven’t found it already.
The blockhouses can not be destroyed by any succesful roll during execution of this order. You will only be able to kill German tanks, infanty and artillery.
You will likely have noticed the naval bombardment card which allows you to only target blockhouses. Later during the Alles conduct combat card you will be able to destroy anything hit -tanks, infantry, artillery or blockhouses.