Looks like surface oxidation. The discoloration follows the scale contours, hence only occurs on the area of the blade most exposed to air, moisture, etc. Flitz is an awesome, very fine polish that simply removed that very thin surface layer of oxidized metal. Don't be surprised if the same oxidation pattern returns.
: Advocate, Slysz Bowie Ti, Southard Black Blade, Stretch Carbon Fiber w/ZDP-189, Fluted Ti Native5, Terzuola Starmate, Terzuola SLIPIT, Leaf Storm, Gayle Bradley, Roadie, Chaparral CF, Chaparral Ti Stealth, Des Horn, Stretch FRN (Blue) ZDP-189, Centofante Memory, Military Black Blade CE, Delica4 FRN CE, Endura4 FRN Black Blade CE, Assist Orange FRN, Manbug ZDP-189, Ladybug 3 FRN SE, Delica4 Blue NLEOMF, Moran Drop Point BB, et al.
Spydie Fanatic #179