Is this possible at all? What about specific tilesets, namely Twilight? I tried quite a few fully black tiles from the tileset index but none were passable no matter what.

Any other way (except modding) to get a walkable pitch-black tile would be cool too.

Wiki:Null Terrain

Oh daaaaamn, thanks a bunch.

Sorry for posting a bit late (I'm assuming a few days isn't necroposting), but I would like to add something:

It said that units cannot walk directly onto null terrain. This is not true.
Units can walk from creep to null terrain, by my experience (although for some odd reason they have problems walking off the southern edge of the creep). For the record, I tested it in I dont' remember what tileset (I think jungle), and the null I used was the default null terrain (the kind you get when you tell the map to start out with null terrain). Also, creep on null does not retract when it's source dies, so I think you could make a bunch of zerg buildings (ie, spires) and then kill them with a trigger, and the creep would still be there. Perfect for generating little shapes in the null =D. Creep's also perfect for rounding off the null and making it less blocky.