The terminology "black flowers" is used loosely to refer to the darkest blooms available to growers. Similarly, there are plants with dark foliage termed "black," such as black shamrocks.
If you look closely, you'll usually see that so-called "black flowers" are such a deep purple that they just appear black. I show pictures of such specimens, as well as an example of a plant with dark foliage, in this photo gallery.
|Purple Morning Glory: Almost Black Flowers||Dark Coleus Photo||Dark Foliage of Ajuga||Edens Magic Giant Bergenia Photo|
|Picture of Sand Cherry||Shrubs With Dark Foliage: Smokebush|