The black background isn't something defined in Ruby code. Every time the game needs to draw (essentially every frame), it clears out everything with black then draws the sprites. It's built into the game engine. Instead, you have to create your own background with a custom sprite.
@bg_sprite = Sprite.new
@bg_sprite.bitmap = RPG::Cache.picture('bg_image')
Showing the map as the background is just creating a new Spriteset_Map object, which Drago's script does.