You can easily dye cotton batting. It's often dyed for use in art quilts, but in smallish pieces and for the surface of the quilt, not the inside. It won't turn true black, but it will certainly be dark enough not to worry about when used in dark quilts if you can manage dyeing the larger amount. Rit dye has some info that may help you decide whether to try it. Make sure it doesn't bleed afterwards!
http://www.ritdye.com/diy-projects/q...cotton-batting