Here are some flowers with dark historical meanings. They are suitable for any enemy's wedding or funeral and are lovely additions to an anonymous bouquet to let them know someone's been thinking a whole lot about them:

asphodel: my regrets follow you to the grave
basil: hatred
begonia: I have dark thoughts about you
bilberry: treachery
cistus gum: you will die tomorrow
coltsfoot: justice will be done
crowsbill: envy
crowfoot: ingratitude
dahlia: instability
dark geranium: you are unjust
dogsbane: deceit and lies
enchanter's nightshade: witchcraft and sorcery
fig: strife
hellebore: calumny
hortensia: you are cold
Judas tree: disbelief and betrayal
lobelia: malevolence
lotus flower: tainted love
mandrake: horror
meadow saffron: your best days are gone
oleander: beware
rhododendron: danger
saffron: be wary of success
scarlet auricula: greed
St. John's wort: animosity and superstition
tamarisk: crime
trefoil: revenge
wild tansy: we are at war
white catchfly: betrayal

Sources: Murder Ink edited by Dilys Winn and The Language of Flowers by Margaret Pickston