To clarify, I think my misunderstanding, and what may have confused others, is that I used the mask from within the effect, as opposed to creating the mask and duplicating the clip, which seems the be the alternate (traditional?) way of using the mask. When the mask is created inside the effect, its state (on/off) cannot be animated. I'd assumed it was kind of "a part" of the effect so could be animated, and was wrong about that, or at least that's my take, so far!
It was something pretty simple/silly, where I was playing with 3 angles of a group of 3 people, and I took turns in the angles animating the b/w effect, so that it would, essentially, highlight one of the people by leaving them in color, while the background and other 2 folks would all go to grayscale. At the end of the short video, I wanted to simply use the center angle and animate the b/w so the entire thing would fade to b/w, but the mask could not be turned off for just that portion of the video, i.e., via animation. Blading the end part was a good solution in this case since I had a transition going into that angle anyway.
