It could be a primer, but that's probably decarb. Heat treatment tends to cause carbon to migrate from the steel at the surface; that's decarburization. The first round of Competition Finished INFI knives left the decarb on the knives (the CF trade-off was that you got a fully convex edge but they didn't finish the blades to reduce production costs). Unfortunately, decarb is a thin layer of very high-carbon steel residue that is very vulnerable to corrosion, so on the last batch of (BWM) Competition Finished knives, they also made a point to remove the decarb -- those CF BWMs are OUTSTANDING knives.