No worries this story line was most likely completely abandoned and we will never hear again of the Black Garden in the sequels. T
hough after learning what we did from Oryx and the Books of Sorrow, in the context of Destiny 1, I figure the Black Garden and the Heart are just the Vex's attempt of making a Throne World and an Oversoul respectively.
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Actually, I think the garden may end up playing a big part.[spoiler]I've theorized that the vex (more specifically the Sol divisive) will be responsible for summoning back the darkness itself. The only place I could see that happening is in the garden.[/spoiler]