A lot of things can cause it.
I can attst to heat treatment changing how steel blues. If I do a blade in such a way that I only harden the edge by using a controlled depth quench, the hardened edge will blue much darker than the spine area, if I blue it. If I harden the whole blade then draw back the spine of the blade to a softer temper that blade will color differently than the edge quenched one. If I use 1095 high carbon and 15N20 (3%+ nickel) to make damascus the high carbon will blue deep and dark and it will hardly effect the high nickel content 15N20.