The minor downside to a carbon-steel paella pan is the little extra care it requires. Like a cast-iron skillet, it’s prone to rust, so it should be seasoned before the first use and carefully cleaned and re-seasoned between uses.
Carbon steel pans should not be put in the dishwasher or they will rust. The easiest way to clean is as follows: Fill the dirty pan with about 1/2 inch of water and let it sit for a couple of hours or until the residue is easily removed. Then pour out the water and residue. Follow this by washing the pan by hand with soap and water, and then immediately dry (never let pan air dry).
Finally, rub a small amount of vegetable oil on the pan to seal and prevent rust. If you forget to seal the pan and it does rust before your next use, don’t worry. Just remove the rust with a bit of steel wool (it comes off easily) before using it.
Discoloration (turning blue-black) is normal and is an indication that your pan is ‘seasoned’ and has been properly cared for. Here is a picture of my Paella catering business pans. As you can see they discolor as the pan seasons.
Enjoy the video on how to Clean your paella pan.