You could also try 'setting' the black fabric with white vinegar before using. If you prewash, that is. Haven't done this myself but a friend swore by this for her black clothing. It makes sense. Kind of like setting the colors on Easter eggs when dying with the old hot water mixes.

Black is more reliable now than it was during the civil war. Black used in their quilts was usually wool or silk because they would hold the dye and not fade as quickly. If you are going for authenticity you might want to re-think black cotton.

Best bet is to go with the Kona Black.

I use Micheal Miller Jet and wash it in Synthropol to help set the dye! I have had Kona turn purple in our water!

Black will fade if left in sunlight too often. Like to close to a window.

I love knona too!

I love knona too!

I use black made by Blank. It will be the same color bolt from bolt. Some manufactors empty all the dyes from the day into one vat to make their black. Blank does not do that. They have a true black dye.

if it's dyed fabric, yup, it's gonna fade...no getting around that--same as any other color...

This was my thoughts, also. They leaned toward brown & indigo (inky/navy blue sort of) if they wanted dark cotton fabric. If you used actual civil war repro fabrics to make the quilt. If you just made the blocks with whatever fabric you wanted, then the Kona Black would also be my suggestion.