My stainless clad Takeda gyuto has been developing a patina which allowed me to see something odd with the blade construction. Before the patina you couldn't notice much difference between the core steel and the cladding (non KU part). Now it's almost as if there is some core steel which refuses to develop a patina. I thoroughly cleaned the blade so am certain it isn't some sort of lacquer or coating preventing oxidization. It sort of has a distinct appearance from the cladding too so it isn't jigane (it seems to be between the cladding and core steel). It only really seems to be at the heel and at the tip.
Left side at towards the heel:
Left side at towards the tip:
Right side at towards the heel:
Any idea what it is (flux?) and if it may be problematic? I'm looking into a rehandle for this knife and don't want to "invest" in it if there is risk of an issue like de-lamination or something.