Performances have changed. Everybody used new technology. From then on audiences were not going to accept a band on a black box with a bit of steel and tarpaulin over their heads and a couple of big white sheets at the side of the stage with pictures.

1. It seems to me that "black box" it's just a stage, dark stage, without much light.

2. Steel and tarpaulin - is it referring to what the stage was made of (it was over their heads)? I know what's steel, but tarpaulin associates me some material like "sheets"

3. Big white sheets - does it just refer to the material people have in beds and they would put some pictures on them, or maybe there would be something like a slide show on them?
I agree about the black box in this case, although in other contexts it's a divice where you put something in and get something out and don't much care what happens inside.

No more homemade sets, a la Woodstock.

Tarp is canvas, or tent material. The "roof" would be a steel framework with canvas against the rain.

"Big white sheets" could be cloth (eg. bedding) or some other material, like poster boards. I think the pictures would be art work or blown up photos. It seems like if the images were projected he would say so.
I got it. Thank you!