The first questions is why do you have black bars? There should never be black bars in your project. If you want to work in a different aspect ratio then set your project up for that aspect and edit in that aspect and then have the black bars added when you render to a different aspect. This will keep any FX from affecting the black bars.
If you insist on working with black bars in your project you could use a Mask and set it to limit the FX (i.e., Apply to FX):
But that's a lot of work. You could also create black bas with generated media and drop them on a track above your entire project but it's better to not have black bars in your project at all.
Easier still, just crop your video event as you've already done and create a track beneath it that is solid black. Or you could create your black bars in Photoshop, save as a png and place it in the track above. Lots of ways to do this.