Carbon steel is often used in knives to give them an unbeatable sharpness. You do however need to give them just a little extra care to keep them in top shape.
You will find over time that the blade changes in appearance slightly, this is call a patina. Don't worry, it's a normal property of carbon steel as it reacts to the air and the chemicals you expose it to in the food you cut. This patina is unique to every blade and gives your knife a little character, think of it as ageing like a good leather jacket. Most of our knives are laminated knives.
Most Japanese knives are made up like a sandwich with a stainless steel on the outside and a high carbon steel at the core which forms the cutting edge (some are made entirely from a single piece of steel and is referred to as mono steel construction and so a patina will cover the entire blade).
Stainless clad knives will only get a patina along the cutting edge which can surprise some people who've not experienced carbon steel before.
Again this is totally normal however you might not find it desirable. If that is likely to be the case (and you want everything shiny all the time) then look out for knives that aren't made with high carbon edge (such as those made with VG10 or R2 steels), please be warned, it will happen.
Rust spots on your blade? Don't worry!
When you use your knife, make an effort to wash it straight away (not in the dishwasher!) and dry it immediately. Don't leave it on the draining board over night or sat in a pool of water. This risks the knife going beyond a simple patina and instead starting to rust.
Along with a patina, you might sometimes see rust spots on your knife but don't panic! Again, it's a perfectly normal property of the steel and you haven't done anything wrong and there's nothing wrong with the steel itself.
It can be a little unnerving if you're not expecting it if you're new to the reactive steels used in some Japanese knives.
You can just clean any rust spots off with a brush or non scratch dish scrubber. If they're particularly stubborn, you might want to squeeze a little lemon juice on the spots and leave it a few minutes before scrubbing.