A carbon steel knife should develop a nice blue/black color. Your blade should get darker as you use it. This is due to the formation of magnetite (Fe3 O4), the black oxide of iron. This is the same thing you get as you use your cast iron skillet or dutch oven.

It is a good thing. This thin layer of Magnetite gives some protection against the formation of red iron oxide (Fe2 O4), i.e. plain old rust. Note: that is SOME portection. Leave the blade wet or use around salt water and it will still rust.

If anyone says anything about your knife, tell them your knife has a classic cold blued finish that you just cannot acheive with plain old stainless blades. <img src="/images/graemlins/grin.gif" alt="" />