I tried reproduce Barry-H negative boolean approach.
But already have one additional question.
I dunno how does you reconstruct such a clean boolean shape(blue layer shape).
If we select naked edges and then join them we would have pretty messy curve.
And if we use rebuild curve operator even with Tolerance: 0.001 we would lose straight part of curve.
So i guess that this boolean shape was constructed from original curves and surfaces (light ones).
So, can you please share, how did you do that ?