The traditional way to get a dark background in the garden is to plant a row of evergreens, and that's okay sometimes, though, if light shines on the evergreens they lose their "darkness" and if they have glossy leaves, they can even sparkle distractingly. And maybe you don't want to dedicate space to foils!
It isn't always possible, but sometimes you can position flowers so that the sun lights them up while the background is shaded. Large flowers, like this rose explode against a shady background. Smaller flowers, Salvia gregii and Verbena bonariensis have worked well for me, sparkle like jewels. I have found that the easiest flowers to integrate into this arrangement are lilies because they hold large blooms up in the air where the sunlight can easily find them. All you need to do is figure out where the shade is.
The Garden in Adelphi is linear on a North/South axis with large green woody plants at the back (South) end. The sun continually lights up the mid-ground of the garden and the large woodies in the back provide a shady background all day so its pretty easy for me. But there are many other ways to achieve this effect; you just have to pay a lot of attention to your individual garden. In the front of the Adelphi garden there is a yew hedge running N/S along the east side of the garden. The sun gradually rising over it lights up the perennials in the garden while still leaving the base of the hedge cloaked in shade. So sometimes, I guess, you can use evergreens and shade.