Subs almost never block movement of any kind. The only exception is that a transport may not offload for an amphibious assault in a sea zone containing enemy subs unless there is also a warship of any type belonging to that power in the sea zone.
However, if enemy subs (and/or transports) in a sea are ignored by the attacker, but then defending air units are scrambled into the sea zone, the defender forces a battle in which the defending subs (and/or transports) will participate.
In cyanight’s scrambling example, the French infantry is prevented from offloading by the German scramble. As it is not the UK’s turn, the UK transport is not participating in the attack. Since there are no attacking sea or air units, no battle will be fought; however, since France cannot clear the sea zone of defending units (the scrambled fighter), the amphibious assault cannot proceed. The net result is that nothing happens, and no units are lost by either side.