This could be, but I think it could also be nothing more than blackwood dust trapped in the coarser grit scratches. I say this because the handle where I have experience was blue dyed maple and black G10, and the G10 looked grey/purple (which I took to be the blue dust) until either 800 or1500 grit when it started to go away.I believe the G10 is actually burned a bit which is causing it to look that way.