Blue line speed = decision speed so there is no 'in between zone.' Failure below: close both throttles, land
All too simplistic and typical of some flying schools. In any case, Blue line speed is nothing more than best rate of climb on single engine with clean aircraft and prop feathered. Another speed quoted in some POH is best angle of climb speed to clear obstacles which still gives you a positive rate of climb although not necessarily as good as Blue Line. There is no published "Decision" speed on light twins.
The so-called "Decision Speed" quoted by some instructors is a personal speed and nothing else. Same as an earlier post regarding "Decision height" for asymmetric go-around. None is published in the manufacturer's AFM/POH so someone in a flying school invents one and lo and behold it catches on and quoted widely as a measured fact. "When ignorance is bliss, 'tis folly to be wise" comes to mind...