Seems to be the rumored "black triangle" military stealth blimp that America has supposedly been using since the 1980s.
Lighter-than-air vehicles held all records for payload, distance, duration, and altitude within the first four decades of the 20th century - even with the advent of the airplane. In fact, save for rocket-powered research aircraft, like the X-15 and the space shuttle, all absolute altitude records are still held by high-altitude scientific balloons.
Big Black Deltas, or BBDs, are considered to be U.S. Defense Department airships. They are so large they can carry massive payloads at low altitudes, cruising at speeds three to five times as fast as surface ships. BBDs are probably powered by electrokinetic/field drives, or airborne nuclear power units. These craft also fly at extreme altitudes, high above conventional aircraft and the pulsing of ground-based traffic control radar.
Elecrokinetic propulsion means that no propellers or jets are used. A hybrid lighter-than-air craft would rely on aerostatic, lift gas, like a balloon. No helicopter-like downwash would be produced. Except for a slight humming from high-voltage control equipment -- and in older BBD versions an occasional coronal discharge -- a Big Black Delta makes no noise.
Given a slew of BBD capabilities -- from silent running, diminished drag, elimination of sonic shockwaves, to operation from ground level to full vacuum -- it seems likely that this would have been considered and developed by now.
If the nuclear powered electrokenetic propulsion is too much for you to swallow, than thrust vectoring propellars like on the Osprey would surely get the job done.
Just my theory.
