The process is the same as rust bluing steel barrels, with an extra step added. The extra step is to etch them in a weak solution of ferric chloride between rusting and carding. In the case of black and white Damascus, they may also be boiled in a logwood chip bath to enhance the contrast. Dr. Oscar Gaddy wrote an excellent series of articles in the DGJ several years ago on how he did it. Perhaps someone with an index can provide the issues containing the articles.