OK - let me put the alternative view.
This idea of "taking the space" is of very limited scope. Didds's original post wondered whether Blue commits an offence by passing through the tackle zone in order to get to the right side of it. The answer is broadly no - as the law stipulates (what we simplify as) a gate only for players who attempt to the play the ball:
[LAWS]15.6(d) At a tackle or near to a tackle, other players who play the ball must do so from behind the ball and from directly behind the tackled player or the tackler closest to those players’ goal line.
Sanction: Penalty kick[/LAWS]
His secondary question asks whether Red is entitled to push, shove or otherwise interfere with a Blue player who is between the Red potential receive and the tackled player wishing to offload. In my view, this is wholly illegal and liable to penalty:
[LAWS]10.4(f) Playing an opponent without the ball. Except in a scrum, ruck or maul, a player who is not in possession of the ball must not hold, push or obstruct an opponent not carrying the ball.
Sanction: Penalty kick[/LAWS]
The situation is a tackle. There is no offside line. Blue is perfectly entitled to stand between the tackled player and a Red potential receiver, with a view to disrupting the pass - just as Blue 10 is entitled to stand right next to Red 10 once they've got their line organised. Just as Red 10 would be penalised for shoving Blue 10 before the ruck formed (making Blue 10 offside), so the Red potential receiver must be penalised for shoving the Blue player out of the way. Blue has committed no prior offence at all (let alone a material one) which the ref can come back to as the first offence. Indeed, there is a very powerful argument to say that the shove should always result in a whistle - either to penalise the shove itself, or to penalise the first offence by Blue that caused the second by Red. How many would allow a Red knock-on to go unpunished just because it was preceded by a Blue knock-on? If you wouldn't let that go, why let Red's violation of 10.4(f) go unpunished just because Blue had earlier committed an offence - particularly one like "taking the space" which is not covered in the law book?