Carbon steel takes a bite stainless rarely matches. It also stains. That darkening is a patina — a record of what you have cooked — not a fault.
The habit is simple: wash, dry at once, and do not leave it in the sink. A drop of food-safe oil if it will sit. Never the dishwasher.
Rust is orange and dusty. Patina is grey-blue and smooth. If you see orange, a nylon scrub and oil will usually bring it back. If you see grey, leave it — the blade is settling in.