Certainly 7 points is not a blow out. At 7 points, you never know if there will be a last second TD to tie the game. And 14 points is not a blow out. With a 14 point lead in the 4th quarter, you still have to worry about the trailing team scoring, and then getting the ball back with an onside kick. A 21 point lead in the 4th quarter is a comfortable lead, and you can be fairly confident that you'll win, even if the trailing team scores to bring the final score within 14. But a comfortable lead is not the same as a blow out (assuming a blow out is a commanding or one-sided victory, not just a comfortable victory).
You really need to get to 28 points to get into blow out territory. But as Rutgers fans know, 28 point leads can be overcome. Also a 28 point victory which was a 21 point lead late in the 4th quarter, with a last minute touchdown, doesn't really qualify as a blowout. But a 28 point victory which is a 35 point lead late in the 4th quarter with a last minute touchdown by the trailing team does qualify as a blow out.
So I'd say that a blow out is a 35+ point victory, or a victory in which the winning team leads by 35+ points at the 8 minute mark in the 4th quarter.