Sorry no one saw your post here- there are a number that look truly black in the garden because they are so velvety they don't reflect the light much. Here's an older thread about favorite blacks.

Here is a link that might be useful: Blackest black irises

black does not exist in iris you've seen the photos if you want to pursue this plant them out of direct sunlight they then appear black you need to check out the iris lovers page on facebook type arilbred iris enjoy the photos