If you are going to keep them black, I would definitely use white flowers maybe all white or mix a light color with the white. Red might be too dark with the black. Take a piece of black construction paper with you when you go to the nursery and place plants against it to see how they would look
Look at pictures of English gardens. They have a lot of antique fountains. urns, pots, cement walls, etc. They are usually grayish in color and are distressed because of their age. I would try for something like that. I would love to see those urns look like they have been in an English garden for years. I love the look of weathered stone.