We have found 16 plants that meet your criteria.

Dusky Cranesbill ‘Geranium phaeum’ Small, deep maroon flowers. Good for groundcover £5.99 Buy
Price Pruned
Dusky Cranesbill ‘Geranium phaeum var. phaeum Samobor’ Small, purple-black flowers. Good for damp or dry shade NOW £5.49 Buy
Pincushion Flower ‘Scabiosa atropurpurea Chile Black’ Deep claret, almost black flowers £5.99 Buy
Moor Grass ‘Sesleria autumnalis’ Perfect for informal edging £5.99 Buy
Double Late Tulip ‘Tulipa Black Hero’ Almost black double flowers £5.00 Buy
Tulip Black Jewel ‘Tulipa Black Jewel’ A tall, very dark coloured tulip £7.50 Buy
Fringed Tulip ‘Tulipa Curley Sue’ Delicately fringed flower petals £4.00 Buy
Triumph Tulip ‘Tulipa Negrita’ A sturdy tulip with rich colour £3.00 Buy
Single Late Tulip ‘Tulipa Paul Scherer’ Possibly the darkest purple tulip £4.00 Buy
Single Late Tulip ‘Tulipa Queen of Night’ Velvety, dark maroon blooms £3.00 Buy
Lenten Rose ‘Helleborus x hybridus Harvington Shades of Night’ Early spring flowers in shades of deep purple £9.99 Buy
Amaryllis Black Pearl ‘Hippeastrum Black Pearl’ Stunning dark maroon flowers £8.99 Buy
Siberian Iris ‘Iris chrysographes black-flowered’ Stunning, almost black flowers. For a damp spot £6.99 Buy
Parrot Tulip ‘Tulipa Black Parrot’ Dramatic, blackish-purple, cup-shaped flowers £5.00 Buy
Triumph Tulip ‘Tulipa Havran’ Darkest blackcurrant coloured blooms £3.00 Buy
Price Pruned
Red Hook Sedge ‘Uncinia rubra’ Wonderful deep bronzy-red foliage NOW £5.99 Buy