Rit will not get you the nice dark, blue dun you are looking for. And as you have already found out, no suppliers are providing Schlappen with the dark blue dun coloration. To get the proper color, you will have go dye white schlappen with Fly Dye's Dark Dun dye. Use vinegar in the dye bath to set the dye, and use 1/4 teaspoon of the Fly Dye dark dun dye. Heat the dye bath to about 160 degrees (about the time you start seeing some steam come off the dye bath but before it starts to form bubbles). Put the white schlappen in the dye bath and let it stay in it for about 15 to 20 minutes. Then dump it into a strainer and rinse with luke-warm water until the water runs clear. Set aside the dyed schlappen to dry and you will have a beautiful dark blue dun spey hackle.
Fly Dye is available directly from the manufacturer and the web address of the company can be found in fly fishing or fly tying magazines. An once of the dye sells for about $7.00 and an once will dye about 2 pounds of schlappen.