There are some pansies that have black or nearly black flowers. Also, hollyhock "The Watchman" has a nearly black flower. If you do a search on gothic gardening, you will find a site that talks about dark plants.

There are no truly black plants (except dead ones) however, those really really deep purples that masquerade as black include diabolo ninebark and purple smokebush. Also there's a "black" tulip" and some near-black aquilegias that might interest you. I also saw a near-black phormium not long ago that had a yellowish racing stripe on it...that could work.

I'm currently in love with Columbine, and 'Black Barlow' is a beautiful dark, dark purple variety. 'William Guinness' is the dark purple, but has a white center.

There's also poppy 'Black Peony'.

Little Blackie Sweet Potato Vine grows very fast and
is quite attractive. Japanese Bloodgrass has a Burgundy
leaves and a deep Smoky Plum colored bloom ( looks like
a fuzzy plume)

I have black mondo grass. I have not had it in the ground over the winter yet, but I hear that it overwinters here.

lOUISIANA IRIS BLACK GAMECOCK IS GREAT FOR BESIDE THE POND. yOU CNA GET IT AT ADAMS FAIR ACRE FARMS CHEEPER THAN ONLINE TOO.