Rust. It’s just comes with the territory when using high carbon steel blades. Let’s talk about it in a bit more detail. The rust pictured in the first photo of the ESEE-AGK is surface rust and should not be a huge concern as it is more aesthetic in nature.
If your blade develops this kind of rust you can remove it with steel wool or even a leather strop with a little time and elbow grease. Some preventative steps can be taken to combat this; one of which is never put a wet knife back in the sheath. Wiping it off before re-sheathing prevents a massive amount of rust and is a great habit to get into. If your knife gets wet wipe it down as soon as you can and let the sheath dry out completely before putting into storage. Treating the exposed steel will prevent it from rusting while stored in humid climates.
The recipe for black oxide blades varies from our powder coated ones in that you need to coat the entire blade. For our powder coated blades, you simply work a rust prohibitor into the area of the engraving and exposed metal on the edge. I often use Red Eyed Hog (https://redeyedhog.com) Bfat as a rust inhibitor. In a pinch I’ve also used light coats of cooking oils, chap stick, and Vaseline.
The CR-2.5 in the photo below was caught out in the rain for a very wet weekend of SAR Training and then left unattended for some time, it looked far worse than the AGK or PR4 also pictured. When I went to use it again, it was covered in rust. I took a little steel wool and strop and with time this is the end result. Super sharp and no ill effects after being neglected for some time and ready to go back into service.