@MrGrowthSpurt, not the author of the photos, but you can get pitch black backgrounds by simply having a darker background and lighting just the plant. Basically there should be a big difference between how well lit the object (plant) is, and how dark the background is. Then you set your exposure for the plant, not the whole scene (or just underexpose if you're winging it) and after simple editing you should have a similar photo.
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