Since I don't think you're simply wanting to delete points above the magenta curve, I don't think you can just simply do bTrim(bLine>mLine) = NaN;
I don't think that will handle the case where the magenta curve bends around and goes backwards. So I'd make the magenta curve into a closed polygon with the upper left and upper right corners of the graph (which you can get with xlim and ylim) and then use inpolygon() to set those inside the polygon to null.
But you didn't attach your points and you accepted that answer, so whatever - at least you're happy with what it does. If not, write back, attach your data, and I'll give you my code.