I tried punching this in on an online DoF calculator which has 1/4" sensors, but it only goes to 1200mm@F/45[gameboy], which is the equivalent of the 70mm@F/4[fullframe] end of this lens. Even then, focusing at something 40 ft away you have a 0.2 ft depth of field, so a background 10 ft behind your subject will be pure blur.
Out of focus blur is a function of focal length and aperture. The sensor size only dictates the crop.
In this case, the lens widest setting with a 1.4 extender is 98mm @ f/4. On a full frame sensor, something 10ft behind a in-focus 40ft away thing would still be very recognisable. Since a crop sensor is just a crop, I don't think it would have a different effect.
The 1200mm focal length equivalent only applies to the fov radius, not to the blurriness.
What's your definition of "pure blur" then?