I'm sorry, I read through my books on powder horns, and can't find any information in either direction as whether bleach can be used to whiten a horn. Staining, yes that was covered, but not bleaching. I have this real nice color photographed instuction book Titled...REACREATING THE 18TH CENTURY POWDER HORN by Scott and Cathy Sibley. Track of the Wolf published it. I'm not positive but I think that they have an association with them, friends or something. Maybe someone there can help you.
Sorry, I tried.......George F.