Minding your knives Carbon steel needs a good bit of looking after and will rust brown or black after use especially  if you leave the knife wet expect to come back to a red rusty knife.  Depending on how long the knife has been rusting there may be some pitting in the steel of the blade.  If the carbon steel is good quality you should find less pitting also it wont be very deep. 

If it is extensive it is likely the steel isn’t of good quality.  Pitting doesn’t look pretty but more importantly it weakens the blade. You can clean the blades with a new Brillo pad (steel wool) and hot water this won’t take the pitting out if it is deep but will clean  a heavily tarnished blade 

 Can you get the pitts out?  that really depends on how deep they go. If you have no success with a Brillo pad you can try sand paper.

 Look for the type used in car body shops (ask in the garage or auto factor) which can be used wet or dry grits 120 and 240 are good start with the 120 to take out the heavier Pitts and finish with 240 keep changing the paper so you are not using a worn piece it just takes longer also don’t work with the cutting edge toward you because the blade will sharpen up with the sanding. If you cant get all the pits out don’t worry the knife should still be serviceable.   

 Dry the knife and oil the blade with some kind of cooking oil don’t use other mineral oils or silicone as this will make the food taste peculiar and is not good for you if the knife is in regular use. If you want to store the knife one way is to oil the blade liberally and wrap it in cling film. 

 Carbon steel knives do need more care but many chefs love it for its ease in sharpening and skinny edge so it cuts very well also these kinds of knives tend to be traditional in design often with rose wood handles and brass rivets so have their own unique charm