I think the black is iron scale as in this example.
http://www.vikingsword.com/vb/showth...highlight=naga
This is the only Asian weapon I have seen use the black as decoration.  All the other I have seen come from Africa,  Somali, Shona, Massai? and last but not least the Congo.  There are many military knives that are black but I think that is for other reasons.  I will post picks of a seme I have with some black as decoration.