To selectively temper the ridge of a blade you submerge the edge side in water and using a blowtorch you start at the tang side of the ridge and heat that spot until it is peacock blue. As soon as it is blue, you start moving the flame slowly downwards towards the tip of the blade drawing the blue colour along the ridge. Do not blue temper the edge or tip because then hardness is lost and will result in a blade not holding an edge.