There is some interesting concept art from early in the developement that implied that the black garden was contained within a large sphere... http://www.forbes.com/sites/insertcoin/2014/09/29/destinys-lost-story-reveals-a-very-different-game-prior-to-launch/ (2nd image if you scroll down) As if the garden weren't always black but had been made that way after IT defeated the gardener - this ties into why defeating the black heart of the guarden would allow the traveler to begin to heal..perhaps?