'Negritjanka' is the darkest flowering one I have ever encountered. In my garden, the coloring is a very deep purple. It's not all that uncommon a vine and should be available through online vendors. Check out the list of clematis sources addressed on a previous thread. Both Joy Creek and Silver Star carry it.

Out of curiosity I went to Clematis on the Web and searched for the terms black, dark and night. Judging from their images, in addition to 'Negritjanka', 'Black Prince' and 'Dark Eyes' are all quite dark.
'Black Prince' is available from Joy Creek Nursery.
http://www.joycreek.com/ref-cl.htm
They don't have a posted picture, but use the link below to see it on CotW.
'Dark Eyes' is listed at Silver Star Vinery, an excellent source.
http://www.silverstarvinery.com//vines.asp?letter=D
Silver Star also carries 'Negritianka', which may be the same as GG suggested above with a different spelling of the name.

I have a plant that I bought as a 'Niobe' that never gets as red as the photos; it's an extremely dark maroon (to my disappointment since it's too dark for its location.)

Here is a link that might be useful: 'Black Prince' - Clematis on the Web

Romantika is dark too