I had some black Hollyhock last year, "The Watchman", and they are so pretty! It really does look black, but when you look closely it's like a velvety, dark purple, burgandy kind of flower. It would look great against a light backdrop. I didn't realize it when I planted them at the mailbox. They did not come back this year though, so I will have to plant them again.
Does Black Current shows up black looking? The Black Current Swirl is very pretty!
Here is a picture of "The Watchman" from last year: