First off are you sure you want blue for a knife? Typically that is considered too soft...most makers d blades to a straw/gold color (400-450).
Definitely leave it in for longer times. Most makers leave it in for 2+ hours, for 2-3 cycles. At 20mins there is a very good chance you are only getting the surface hot, and not all the way through (might explain your splotchy colors).
No water quench is needed after tempering, just let it air cool. Quenching is done after hardening....did you harden the steel prior to tempering it?