You can turn cookies and pastry black with black food colouring/icing colouring. I made Black coloured lemon flavour Bauhaus cookies once ^_^
We have an ornamental black grass in our garden,I think it's called Mondo Grass or something.
You, can get black Agapanthus' , black Geraniums, black stemmed Dogwood, black Bamboo ( we have this ) black Elder, Budlea comes in bright purple and a darker "black" variety, smells GORGEOUS and attracts hordes of butterflies, Phormium is like a big black grass/palm , the Japanese Maple is a smallish ornamental tree with dark purplish red leaves and a dark wood/stems, make sure to keep it out of the wind though, they get windburn really easily, ours died

We have a black variety of Sycamore tree round here as well.
Cyclamen has dark leaves, and pretty red flowers, Black pansies and violas are easily available, Privet is a dark leaved shrub and can be grown into hedges, Peonies have fairly dark green foliage and those gorgeous huge deep red flowers, you can get black or very very dark red Fuschias, and virginia creeper turns a gorgeous deep red in autumn, as you can see to the left of my head in
THIS picture of me. That's on our back fence. It's lovely.
If ever I manage to get a house my garden will be crammed with black plants.