Might be some metal swarf from the final hafting/buffing of the butt of the knife, embedded into the wood via the buffing wheel. It's pretty easy to stain or discolor natural handle materials (wood, bone, stag) like that, during sanding or polishing of adjacent metal areas. Or it might just be a natural dark spot. Either way, some gentle sanding and frequent magnified, lighted inspection while sanding might reveal how deep it goes into the wood. Sand a little bit, then inspect under brightly lit magnification to see if the edges of the dark spot begin to recede with removal of some wood. Start with fairly higher grit (maybe 400-600), maybe dropping down to 320/220 if it looks like it can be removed this way.
Might also see if some mineral spirits, alcohol or acetone on a rag might remove some discoloration. Use a clean, white rag; if some of the dark stuff comes out, you should easily see it on the rag. That could reveal if it was dirty oil, or dirty metal polish or something similar. If no color comes out, it may just be a natural dark spot in the wood, or a burned spot from lingering too long on the buffer (back to sanding, for those).
I'm not seeing any of it on the side-view pics of the handle. So I'm assuming, whatever it is, it doesn't go very deep into the grain at the end of the handle. On that alone, I'd think sanding should probably remove it pretty easily.
David
