Sorry, I know this is a simple trick, but I don't know it.

I want to darken a rectangular section of a clip, so that I can put a subtitle in it. The video is woods and it changes too much to use a dark/light font.

Thanks!

Just put a rectangle on a title using the Adobe Title Designer (F9)

Then set the color and opacity of the rectangle. Put the title over that.

That completely worked, thanks much Steven!