Paeonia delaveyi makes it into May’s list for its tempestuous dark foliage. For this month, just right now in the black or darkest maroon leaf palette there are some cool perennials. Heuchera obsidian is, I think, the darkest of this genus. Grow it with Dicentra spectabilis ‘Alba’ behind it) Anthrisus sylvestris ‘Ravenswing’ – black cowparsely (try with Iris variegata pallida). Polemonium caerulea Bressingham Purple is Jacobs ladder, blue flower and stalk and leaves shot with a black colouring. Pair it with shocking orange Geum Borisii.
Corylus avellana Red Majesty is the twisty corkscrew willow but with dark maroon leaves. A good small specimen tree for a courtyard garden.