"purple-black foliage, stalks and spathes with a turquoise bloom" I hadn't thought of that particular combination - I'll get to work on that, but somebody else might have to take the baton for the final leg ;0)
In all seriousness though, I agree that turquoise sure can be done. Just look at the strides Don Spoon is making with his SDB's (pictured and discussed in the latest Iris Bulletin). I'm hoping that the faint glimpses of turquoise I'm seeing, will advance at a little faster rate, since I'll be working with diploids. All that I've noticed so far has purely been the result of accident, since I didn't start intentionally crossing with turquoise in mind until just last year.
I think the "purple-black foliage, stalks and spathes" part is going to happen a lot sooner. I've already got highly "outbred" (heterozygous) seedlings that show good _expression_ of the different purple plant traits. Next year, I'll start intercrossing and inbreeding these lines to see what double dosages of some of these genes might do. With the three generations of crosses that I've been working on this project so far, I've yet to intercross two strongly PBF irises, I've mostly been intercrossing strong PBF with seedlings having red-margins derived from Iris suaveolens var. rubromarginata.
Chances are... my first all purple-foliaged iris will also have dark purple flowers because the best seedlings I've got in both the tetraploid and diploid lines just so happen to be dark purples and also happen to pass that color on to many/most of their offspring. But I really like your idea of turquoise! I also really liked the contrast of yellow-orange flower against dark spathes like the picture of 'Captured' that Vicki Craig sent me off-line. Really pretty!
I'm really looking forward to growing an iris with purple-black foliage, stalks and spathes with a turquoise bloom. How long do you think it's gonna take? I'm not getting any younger! If the die hadn't already been cast, I'd probably have tried for that turquoise or blue-green color. I've some growing I didn't toss because the color was present. But they haven't cooperated and I haven't given it a lot of attention because I'd already set myself in other directions. I do think it can be done with enough work.