I have room on either side of my deck in front and I would like to do a yin-yang theme, black and white, white flowers are a dime a dozen, but black flowers are more difficult to come by. I'm in Zone 5. I found these but would like more suggestions please.

Black Irises
Go to the Irises database, Select for Characteristics, then for color. I'd also narrow it down to, say, Tall Beardeds to start.
http://garden.org/plants/group...

Asiatic lilies also.

Black Wizard lily: Lily (Lilium 'Mapira')
Netty's Pride: Lily (Lilium 'Netty's Pride')

Black Mondo Grass (annual here but a nice annual ): Black Mondo Grass (Ophiopogon planiscapus 'Kokuryu')

Tulips, lots more than I'm listing below.

Queen of the Night: Single Late Tulip (Tulipa 'Queen of Night')
Black Parrot: Parrot Tulip (Tulipa 'Black Parrot')

Tropicanna® Black Canna (perennial if in a warm spot in garden against house sometimes): Canna (Canna x generalis Tropicanna® Black)

Here's a good list of annual and perennial plants with dark colors on them.

http://www.hgtv.com/outdoors/f...

Dutchman pipevine would be neat, the flower is or of dark the longer you stare

Ornamental rice "Black Madras"
Salpiglossis sinuata "Black Trumpets"

Is heuchera hardy in 5a? Certain varieties of heuchera like 'Obsidian' are very dark, almost black. Mine is much blacker than any of the pictures I've found online.

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.

The Black Diamond crepe myrtles are hardy to zone 5 but die back every year. Wish I had a good place for them here, they are gorgeous.

Cool! They often die back here, but we had a stretch of mild winters that lasted about 10 years and many in the area grew into trees. That stretch ended winter before last when we had record breaking low temp of -18F, and they all look shrubby again but are still beautiful.

I meant to comment on the black petunia in the opening post- I saw them at a garden center this spring and they really look black. I had to stare at them, no hint of purple to my eye.