I checked the '79 docs from my FFE CD, and it's an all-or-nothing field in the description:The original black globe in 79 High Guard was very much like the Langston Field.
Force Field generators project a barrier which prevents transit of anything but undifferentiated energy. This energy is absorbed, giving the field a black globe ap-pearance, and stored in on-board capacitors. Because the barrier prevents all transit across it, a ship with its black globe force field on cannot maneuver nor can it com-municate or detect enemy forces. In addition, if the field is overloaded, the drives may detonate, and the barrier drop, rendering the ship vulnerable to enemy fire. Ships using black globe are quite vulnerable if outnumbered, as they can be attacked easily when the globe is turned off.
It would also prevent any sort of laser morse code (though arguably it mentions nothing about being able to sense any energy being added, but I think the v5 Traveller explanation was done for the coolness factor rather than keeping with the idea. For something that is supposed to absorb all energy, using a laser to communicate through it seems a bit of a cheat. Langston fields actually allowed for penetration, albeit at very, very low velocities, thus most weapons were rendered pointless.
The idea of flickering got added in a later edition, as were white globes.