I’m not sure when PB&Js became less awkward but the first recipe for PB&J appeared in 1901 and called for currant or crab apple jelly.

National Peanut Butter Board

Turns out Tasting History has a fun video on this topic.

Whether stainless or carbon (or ceramic), you should clean your knife as you work. Doing so will prevent it from rusting and build a patina (if carbon) which will further help prevent rust.

If you want to force the patina, this video by Knifewear might help.

Sharp Knife Shop has a good video about patinas.