We have found 23 plants that meet your criteria.

hollyhock
Alcea rosea 'Nigra' hollyhock Tall stems bearing near-black flowers £1.49 Buy
granny's bonnet
Aquilegia vulgaris var.stellata 'Black Barlow' (Barlow Series) granny's bonnet Striking dark purple, double flowers £7.49 Buy
granny's bonnet
Aquilegia vulgaris 'William Guinness' granny's bonnet Unusual purple-black flowers £7.49 Buy
dahlia bulbs
Dahlia 'Karma Choc' ® dahlia bulbs Opulent chocolate-scented flowers £3.99 Buy
dusky cranesbill
Geranium phaeum dusky cranesbill Small, deep maroon flowers. Good for groundcover £7.49 Buy
dusky cranesbill
Geranium phaeum var. phaeum 'Samobor' dusky cranesbill Small, purple-black flowers. Good for damp... £7.49 Buy
Siberian iris
Iris chrysographes black-flowered Siberian iris Stunning, almost black flowers. For a damp spot £8.99 Buy
bearded iris
Iris 'Superstition' bearded iris Purple-black flowers £8.99 Buy
spencer sweet pea seed Beaujolais
Lathyrus odoratus 'Beaujolais' spencer sweet pea seed Beaujolais... One of the darkest forms £1.89 Buy
lily bulb
Lilium 'Black Beauty' lily bulb Stunning dark red, turkscap flowers £3.99 Buy
lily
Lilium 'Dimension' lily Brooding and sumptuous colour £5.99 Buy
New
lily bulbs
Lilium 'Sixth Sense' lily bulbs Brooding colour for the summer border £2.99 Buy
black-eyed Susan
Rudbeckia occidentalis 'Green Wizard' black-eyed Susan Bright green sepals with black centres £6.99 Buy
coneflower
Rudbeckia occidentalis 'Green Wizard' coneflower Extraordinary and exotic-looking flowers £2.99 Buy
black elder (syn. Black Beauty)
Sambucus nigra f. porphyrophylla 'Gerda' (PBR) black elder (syn. Black Beauty)... Dramatic purple foliage £10.99 Buy
sweet scabious
Scabiosa atropurpurea 'Black Knight' sweet scabious Opulent colour for the border £2.99 Buy
nasturtium
Tropaeolum 'Black Velvet' nasturtium Sumptuous colouring £2.49 Buy
red hook sedge
Uncinia rubra red hook sedge Wonderful deep bronzy-red foliage £7.99 Buy
beard tongue
Penstemon 'Raven' beard tongue Dramatic, deep purple, foxglove-like flowers £6.99 Buy
daylily
Hemerocallis 'American Revolution' daylily Large, deep blood-red flowers in summer £8.49 coming soon