Whichever method you use be sure and get your blade thoroughly cleaned before you begin. I don't know how big of a knife you are working with, but one very simple way to do it is to stick the knife into a potato and leave it there overnight. The next morning take it out and wash it off with soap and water, and apply a thin coat of oil and you're good to go. Another easy way to go is to get you a bunch of onions out and start slicing them up. You could also use lemons, oranges, limes, strawberries, etc.. I have had good luck coating the blade with mustard and letting it set for anywhere from 2-5 hours depending on how dark I wanted the blade, and then wash it off and look at it. If you want it darker, then repeat the process.

Something else you might want to consider is bluing the blade. Just like on your firearms, and it is very inexpensive and easy to do. You can make it dull and non-reflective, bright and glossy, or anywhere in between.It is easy to touch up as it wears, but it is not going to be extremely durable unless you have it hot-blued.