Images in this case are cross origin (they come from another website, so not an image on Codepen), in production with your own images on your server, images will actually be rendered in the blur effect, how cool is that!
Unfortunately html2canvas doesn't appear to just output a context instead an element, on top of that Stackblur doesn't accept literal canvas context anyway, so I gues it will have to rely on the dom. which is just a little niggle really.
I think you could take any effect that could be on canvas and apply it, for example those fancy visualizations you always see on the front page, imagine adding that to a ui effect!