Rule 18 applies as one of the boats is in the zone and none of the exceptions are true.

Rule 18.2(a) does not apply as the boats are not overlapped.
Rule 18.2(b) does not apply as the boats are not overlapped (the first 'if') and first boat to the zone is not clear ahead (the second 'if').

So at the time of the diagram, neither boat is entitled to mark-room.

When Blue turns to head toward the mark, it is likely that an overlap will be established.  At the point in time when that happens 18.2(a) will apply and Blue will be required to give Red mark-room.

That's a great scenario.  Some of us rarely think about boats not being overlapped under 18.2 despite arriving at  the zone 'together'.  Makes you think it through and understand rules all the better.

Kind of fascinating that 18 applies, yet no sub-part of it applies.  So in the end, I conclude that no parts of it apply at that moment.  A bit confusing, but still, I'm constantly in awe at how well though-out the rules are by now.  They mean exactly what they say, and rarely (maybe never) more or less.

And I agree with what John says about how things will likely play out.
-Vince