Black hollyhocks are very dark and look black in some light. Colorblends offers a blend of tulips they call "Black Tie" with a gorgeous photo of a thick mass of 'Queen of the Night' and white lily flowering tulips. That photo has always made me want a black and white bed.
A cool plant for such a bed would be the white flowered crepe myrtle from the Black Diamond series, although I doubt they would be hardy to zone 5.
