I want to make the Toussaint Knight Armor (Rycerska turniejowa zbroja), but to make it the recipe shows I need a Dark Steel Plate (Płyta z Ciemnej Stali), but... there is no such thing! I can't make it, I don't have a recipe for it, the armorers/crafters don't have in in stock at all! I crafted the Chestplate with a Dark Iron Plate (Blacha z Ciemnego Żelaza), but there is no way to acquire the Dark Steel Plate.

Is this some kind of omittance or is the quest for that particular piece of armorcafting lore hidden from me for now? I wanted to have the set as my "go to" armor for fights with humans...

The diagram could be in any loot chest (they're randomized), or some blacksmiths/armorers might have it in their shop inventory. Sorry I can't be more specific.

You could also try dismantling unwanted armor you have in your inventory. The dismantle window will tell you, before you actually dismantle the items, what components you will get, so there is that option.

And double-check grandmaster smith in Beucleur, he must have all new crafting ingridients in store.

Dark Steel Plate does exist, and is relatively common -- you can get it while doing Keira Metz's personal quest. It is crafted from Dark Steel Ingots.

Dark Steel - yes. But Dark Iron? The Ultra Master Armor/Weapon-smith in Beauclair doesn't have those. It's just so bizzare... :/

Could you try dismantling dark steel into dark iron?

You said you had Dark Iron; you crafted the chestplate with it. It was Dark Steel you said didn't exist, but it does. You should make your mind up.

There are two sets; the Toussaint Knight set, and the Toussaint Knight Tourney set.
The first requires Dark Iron Plates for its chestplate, which (apparently) don't exist, and Dark Steel Plates for the rest of the pieces, which do.
The second is the other way around, requiring Dark Steel for its chestplate and Iron for the rest of it.

I just got the Tourney chestplate and the rest from the basic set. They are pretty much the same.

Plates (no matter what kind of material) are processed pieces made of ingots of that material. Most smithies will sell them gladly (the ingots).

ROD2k got me. And i see that it's not just me, but it actually is missing something. Phew, at least.

(sorry if I mistaken the Dark Steel/Dark Iron and Tourney Armor names - I play the Polish version and reverted to the English one just to check the names of ingredients for this post)