Not quite sure why the velvet border is so popular, but I still have a bit to learn.
On my DW screen I do have dark curtains (velvet like material) on the left and right, sort of 'theater style' effect, loosely defining the projected area, but the top and bottom have no borders - the top and bottom of the picture just ends about where the DW screen ends. And, to my eyeball, I don't seem to miss not having added a black border. The wall behind is a dark flat blue.
I don't recall anything much different in the commercial theaters. Is the velvet border in other room situations a needed requirement?