Diggit I really like your seedling nice color !!! and this sounds like a really fun project
I just went through my pictures from the West Coast and I came up with 2 that I would definitely try that a few growers have used recently to produce some darker plicata's. The 1st is (Storm Track) it is one of Kieth's it has produced( Grapeitizer & Penguin Party) both from Mid America and intro'd over the past few years both are darker purple plicata's it also has 2 other plicata's from it one of them was (Tunnel Vision) if you pull up the pedigree of it there is one flower in there that stands out called (Blackout) this plant is used over and over again to produce that almost black color in Iris some of its kids are Oreo, Midnight Oil, and one of my favorites Nobel Gesture has it in the pedigree 3 times so I would try that one and see what happens it has a impressive pedigree of dark Plicata's.
The next is one I think you have or at least you have a excellent picture of it in the database is called( Inside Track) it has produced the two plants Kent mentioned above (Circle Of Light & Wedding Night) so it throw's dark plicatas. I found 2 seedlings from it that are dark that are being grown out in Oregon both used it as a pod parent so if you haven't tried it, I would give it a shot maybe (Inside Track x your seedling x Noble Gesture) or another really dark one with ruffles may get you closer to your goal at least that is my 2 cents worth on it. I hope you find something I would love to have a all black Plicata !!
Here are the pics of the seedlings that I found
I think some of the darkest I have seen lately have come from Schreiners they are (Here Comes The Night & Raven Girl)you might throw them in the mix as well they do still have a little purple not solid black but worth checking out.
I think I have only seen one flower that I thought was really close to achieving a solid Black at least some of it was, I have know idea what made it obviously it is a unfinished seedling but it is the darkest color I have seen up close so I know the genes are there somewhere to get solid black.
