The type and even quality of the wood seems to make a difference. I think that the traditional approach taken by the Japanese created a jet black patina and they may not have always applied a finish to it. In pictures that I have seen, the surface is completely burnt making it s pretty impervious to insects and rot. It also acts as fire retardant when used as siding.
--Nathan, TX. Hire the lazy man. He may not do as much work but that's because he will find a better way.