Nemophila Pennie Black looks great in containers.

Here is a link that might be useful: Nemophila Pennie Black

I say black violas (or pansys) and I think z1 is probably the right place for them. Here the heat blows them out by mid-July but they can struggle on and have a second life in the fall. They also reseed. I have done blue black and white in my window boxes which were great. I like your idea though. Maybe I will try these colors in the spring.

This would be a good opportunity for blackish foliage, too.

The pepper black pearl might be good choice for such a container. It has blackish leaves plus the small imature fruit are shiney black maturing to a bright red. This is an ornamental pepper, although you can eat the very hot red peppers. The plant did very well for me in containers.

I even have some seed for black pearl if you want. Just email me at jkkaplan@erols.com. Home Depot in my area sold plants at one point too.

There are some very dark pincushion flowers (scabiosa), that look wonderful in containers.

Really, black flowers usually turn out to be a very dark burgundy, maroon or purple. But have you ever visited this site? It has a nice list for you to mull through.

Here is a link that might be useful: Chocolate Flower Farm

Thanks Loretta! I had not previously gone to their site, but actually read last night that they are a vendor at the NW Flower & Garden show in Seattle. I am going there for two days so will check them out!...Hmmm, my 'want/have to have' list grows...

Barbara

Moose,
Viola 'Bowles' Black' is very dark:

Here is a link that might be useful: V. 'Bowles' Black'