A lasting effect has been triggered. It stays active until a defined end (here, undefined). It does not end because the card triggering it is no longer revealed.
Or did I miss your point?
Yes and no. The problem is that FFG worded the card badly and instead of rewording the card with the errata, they tried to explain why their original wording is not bad (but it is).
As written, the "In order to declare any characters to attack or defend during challenge of that type, a player must declare all of his or her eligible characters" text is either a continuous effect (that therefore ends when the plot is no longer revealed), or a lasting effect without a duration - which lasts for the rest of the game, regardless of whether the plot is revealed or not. By saying that it is part of the "when revealed" effect, FFG made it the latter.
For revealing Black Walls itself, they took care of the infinite duration by saying "…and is active while the plot is revealed…" in the errata. What they didn't do by saying "…or when the 'When revealed' effect has been trigered by another River plot card" is create a duration for the "copied" effect - which should therefore go for the rest of the game again, right?
The only way for that not to happen is if the "…and is active while the plot is revealed…" part of the errata applies not only to Black Walls itself, but also to the "then" effect of the River plot that "copies" Black Walls.
That was what I was getting at with the earlier comment. The errata creates a duration on the Black Walls of "while this card is revealed." The only way to make this errata not stupid (ie, "work") is to extend the "while this card is revealed" duration to the next River plot through the "then" part of its effect, which is what actually triggers the used Black Walls.
Ultimately, FFG should just make the errata on Black Walls "Should say: 'When revealed, name a challenge type. Until the end of the round, in order to declare any characters to attack or defend during challenges of that type, a player must declare all of his or her eligible characters.'"