I had some hardware soaking in vinegar to remove rust. Been in there maybe a week. Last time I saw this bolt it was almost rust-free, so I let it go another couple days. Then it got cold. It’s been around 15F for the last couple days. When I pulled my bolt out of the vinegar, I had this:

This has gone from clean steel to black oxide(?) in just a couple days. The coating doesn’t rub off or clean off with any chemicals at my disposal (carb cleaner, brake cleaner, POR-15 Metal Prep, mineral spirits and lacquer thinner have failed to budge it), so I assume some sort of chemical change has happened. Can anyone enlighten me?

I’m going to assume (again) that once the weather warms up, I can dunk it back in the vinegar and it’ll start going back to clean steel.