This is a sheer guess, but in reviewing the photo of The Black Garden, I believe the place is one massive memorial to something long forgotten and never described by the actual memorial builders. Something that should be remembered but could never be talked about and, as such, the garden is left without any plaque or inscription detailing its origin. Those who know what it represents bear the burden of that memory; everybody else that visits it is innocent, free to enjoy the garden without caring for the sacrifices of others. The walls of each garden patch are tall, and made out of thick rock or possibly metal, lending great weight and gravity to the task of holding the flowers up. As if a great sacrifice was made, and the effort to hold a simple flower patch up somehow symbolic of that effort. I'm going to go one further and guess that the garden is not located in a major population center, it is more on the outskirts of town. Outside of town, separate, possibly accessible by road, but distinctly on its own. Why this? I find great symbolism between The Black Guardian and the memorial site of the SDF in the Robotech animated series. Both featured essentially flower obelisks. Massive structures of great height with life growing out of them. See the link to see what I mean. The white flowers of "The Black Garden" suggest innocence growing out of the dark walls, the dark being death or loss. There's already been a close similarity between the Destiny art and Robotech with the shot of the pilot in orbit with a fleet behind him, I am not surprised to see a second reference. The more's the better as far as I'm concerned. Also - the Traveller sphere. White colour, or white because it's so tall it's in the upper atmosphere where the temperature is sub zero and it's coated in a layer of Ice or Frost?