Well, the proper time is of course to blip right before you let the clutch out, but the overall timing of the whole down-shift sequence should be fast enough that this proper time ALSO happens immediately upon clutch disengagement.
In other words, you should be blipping upon clutch disengagement, shifting fast, and immediately letting the clutch out before you loose the revs of the blip.
If there is some reason you are delaying the shift sequence, then wait to blip until right before you let the clutch pedal out.
2006 McLareghini Bugatterrari, Storm Titanium... installed: air horn, Scroth 4-point ASM harnesses, Sector111 halon extinguisher and mounting bracket, Von Hep exhaust and rear panel delete, Pagid brake pads, red Volks CE28n wheels, Toyo RA-1 tires, Nitron SA coilovers, Sector111 (WorksBell) quick-disconnect steering wheel kit. awaiting installation: Scroth "pull-up" lap belts, Sector111 RTD Brace, Tony's heater bypass mod, and dropped steering rack mounting plates.