I request the ability to change what you see on the space around the boundary of the map to an icon or color from the interface editor.
var/obj/O = new()
O.icon = 'whatever.dmi'
O.screen_loc = "SOUTHWEST to NORTHEAST"
O.layer = 0.1
screen += O
In response to Garthor
And if you want to be more efficient, you can store that object in a global list along with the rest of your HUD.
In response to Garthor
No need for even using a procedure here!
icon = 'xyz'
screen_loc = "SOUTHWEST to NORTHEAST"
layer = 0
client/screen = newlist(/obj/bg)

Of course, you should really do it like Jeff said instead.

Also, this screen object won't actually override the blackness of what you'd typically call "off screen", in other words, outside of your view, but rather the blackness off the edges of the map.
In response to Garthor
I wasn't thinking of shadows at the time but the space around the boundary of the map. Thanks for showing me this. I didn't know that layer trick. If you have another solution that would be swell. I will edit my original post, because it wasn't as specific as I thought it was.
In response to Garthor
It's worth nothing that that method won't work in isometric mode, but it's a good method for topdown mode.