what is the disadvantage of blended wing body ,that they not used to achieve more than subsonic speed?
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).