That's completely normal. Just about any non-stainless steel will discolor/stain if it comes into contact with blood. If the knife is cleaned and dried quickly after the task is complete, you can usually clean it with warm soapy water and something like #0000 steel wool. In some cases it will remove the stains, but most likely not completely. Whatever you do, DO NOT allow the blade to sit very long, or overnight without being cleaned/lubed.....otherwise blood will literally pit the blade, and the only way to get that out (provided its not too deep) is to refinish/sand the blade. The same thing will happen if you cut anything with and acid or alkali in it......oranges, tomatoes, peppers, or just about any type of meat. The best advice I can offer is to use a good quality paste wax rather then oil on carbon steel blades.......it won't eliminate the staining, but it will significantly reduce it, and give you a long period of time to clean it before rust/pitting starts.
