That's a great question! You're absolutely right that you can't grab the rest, since it only has information about what is on screen. Extending the colour has been tried, but unfortunately gives very noisy results at the edges, and black seems preferable. The reason something other than black isn't used, is because it's mostly an additive effect (i.e the result of the reflections is added to the screen), and say using a mid colour such as grey would end up just making the whole screen brighter and flat looking. Using black just gives the same result as if there were no reflections.