Just as when we say “Italian seasoning”, there was no set list of ingredients, rather just the spirit of an idea that you attempted to capture.
It was a mix you would have on hand, just as we would have curry powder and “poultry seasoning” today. It may well be that various merchants selling it would have kept their ingredients secret.
Thomas Cogan (1545 – 1607), in his Haven of Health, gives his recipe for Powder Blanch: “Also with two ounces of sugar, a quarter of an ounce of ginger, and half a quarter of an ounce of cinnamon, all beaten small into powder, you may make a very good blanch powder to strew upon roasted apples, quinces or wardens, or to sauce a hen . . .”
Other medieval writers suggested ginger, cinnamon and nutmeg with no sugar (which wouldn’t have been very white) or just ginger and white sugar.
Literature & Lore
“White powder-douce occurs No. 51, which seems to be the same as blanch-powder, 132. 193. called blaynshe powder, and bought ready prepared, in Northumb. Book, p. 19.”
— Samuel Pegge, “The Forme of Cury – A Roll of Ancient English Cookery”. 1790.