I am still kind of new to this, but I am seeing a similar problem. I was trying to create a page with just weather related modules. Currrent, forecast, rain map and thought that adding this as a background would work, but I get the black box as well. So, I removed one module at a time until the only one I had left was the background module and still get the black box. I turned on Debug and it said it was ok.
The config is essentially the example on, with sk_ instead of pk_ as mentioned and changed the city.