Remember the ice cube analogy. You are changing the soft pearlite carbon to the hard martensite carbon. Continue heating until you detect a change in color to an orange red color, at which point quickly quench in a container of water. The biggest mistake one can make in hardening high carbon steel is to be impatient and quench the steel before it has all changed to martensite carbon, so watch for the orange-red color. In this state the cutting edge is glass hard and very brittle and could easily chip if used, so another operation is required called tempering.
Tempering softens the hardness of the metal for the use intended.
Here again we are making use of the color of the base metal and, since the hardening of the blade left a black oxidized surface, it is necessary to first polish the blade to the base metal. You will want to do this anyway for appearances if you take pride in your work. If you have a buffing wheel you can bring them to a nice polish as you would your golf clubs. Tempering can be done in good light with a propane torch, but it is risky because the temperature changes so quickly. Use a smaller flame and hold it some distance away from the blade.