The idea is to approach this block by making two pinwheel blocks, as you do for block B-2. Then the clever part: a frame. [hits forehead, gently] - why didn't I think of that? Because I am not Anina, that's why. Above is my pinwheel block, and the frame, freezer paper on the wrong side. The curves are gentle here, and you just need to adjust the points so that it looks right.
Here is the finished block, next to the picture in The Book. I did not plan on reversing the colors but I am very happy in how it turned out. My one recommendation: choose a print fabric that you love, because you and other Janiacs will be looking at this block.
Thank you Anina!