I'll give you that one.
I had a little luck with a boundary blend by recreating the existing surfaces and getting tangency.
The problem is getting control of the flow lines but it is possible if you play with the control points.
In this case, I added 3 sets of control points, the 3rd of which I added in the fly. Once you get the dialog under control, it goes pretty well, but the preview doesn't work so you have to wait until you close the feature to see what it did.
Since the shape is a guessing game anyway, you can probably make something work along these lines. I'd say that adding flow line points prior to the boundary blend will help that process immensely.
