Carbon steel is reactive. Good carbon steel darkens in reaction to acids and oxidizers in food. Rust of course is an oxidation process too, but in the first case you get FeO3, a "black rust" and in the second case you get FeO2, a "red rust."
Black rust is stable and durable. also, if oiled (cooking oil or grease from meats will do fine) black oxide "sets" and stabilizes the surface of the steel -- helping to prevent further rust.
A fine carbon steel knife will and rightly should develop a fine, blue/grey/black patina. You don't want to remove this surface. It's a desirable trait of a fine carbon steel blade.
Nonetheless, carbon steel blades discolor some foods -- eggs for example. I've never had a problem with fish, but I suppose fish might discolor from a reactive carbon blade.
Daphne duLibre is an online name -- It's a pun on a classic European bike race, the Dauphine du Libre. I'm a guy. My knife knowledge comes from a huge collection of hunting, utility, pocket knives, tool steel, and firearms finishes.
I have an old Case 8" French, and a Sabbatier with carbon blades, as well as a sizable collection of carbon blade pocket, hunting, utility knives.