Blended wings have more efficiency than today's wing design,high lift to drag ratio,more efficiency,less fuel rate ,it have lot of advantages so why not they used for commercial airplanes as well as military aircraft rather than UAV.
One of the few production designs that approaches a blended-wing-body was Lockheed Sr-71:
Surprise, with maximum Mach 3.2 it is also the fastest production aircraft ever built.
Because blended-wing-body does not have better L/D at low speed. But generating part of lift with the body has some advantage for high-speed aircraft as it allows smaller wings that is better for high speed (and worse for low-speed).