Thread: Iwasaki Kamisori Repair/Restore

Oz, the black is charred mud left over from the tempering--you've seen it in blade making videos, I'm sure, how the hamon is made by using clay to "insulate" parts of the blade. Iwasaki kamisori received a full clay coating before final heat treating.
If you check this post on my blog: Been and Gone, Sanjou City Post 2: Conception of a Kamisori | Eastern Smooth: The Blog

about halfway down you can see some pics of razors fresh out of heat treat. The clay remains on the whole razor, and is only removed in the grinding.