what is a electric blend door?
An electric blend door is a ventilation door within the vehicles ventilation system that allows the blending of either heated air or cooled air. They used to be called vacuum blend doors because they were controlled with engine vacuum. But now most are controlled with an electric motor.
Tester
It is an adjustable switch to adjust the electricity in a car to AC BC CC or DC. AC was used by the Ancient neanderthals, BC was last used on Roman Chariots and CC was used to make carbon paper, which is seldom needed these days. Most modern cars today only use DC.
Other than that, Tester has it right, BTW there were (are?) cable cable controlled blend doors as well.