I think it's something to do with the sphere being a solid.
I used the Separate tool on it and hid the resultant inner surface - it would then blend at a tangent for me.


Martin (2).

Hi Brian, yeah somehow your sphere piece is not just a single sphere surface but is instead something like 2 surfaces stacked up on top of each other and joined together.

Or I'm not sure if the second surface is exactly stacked on top of it or maybe it's some kind of degenerate "trimmed down to nothing" kind of thing.

At any rate, it's that second unwanted surface that blend is trying to match to - when you blend with an edge that is joined between 2 surfaces is it kind of random which of the 2 is actually taken to be the surface to blend to. So to help control it you usually want to blend between unjoined edges only, with an unjoined edge there is only one surface that owns it and so blend will behave more predictably there.

To get rid of your unwanted other surface here, select the sphere, do Edit > Separate on it, then select all the pieces that you want to keep and then invert the selection and delete the weird unwanted piece.

- Michael

Here is another shape derived from the basketball:

Thank you Martin and Michaelj

Getting rid of the "invisible" other object(s), enabled blend to work "tangentially."

Now to figure out how to create the nice bulge, done with the invisible object... :-)
(maybe a button to swap tangents and normals,...just speculating)

Upon further modification of the "Solid," it seems like maybe the Object name was limiting the selectability
of the various parts of the solid.
Attached is a modified .3dm, in which the Blue denotes two different edges.
There is an inner surface to the spherical solid, in the shape (more or less) of an elliptical cylinder, with curved top and bottom.





Edit: The former Blend that I was doing was actually with the curvy edge of an extruded ellipse:

Hi Brian, yeah it looks like one of the faces of the solid was hidden which is why it looked somewhat weird.

You can have names and styles and visibility assigned to individual face and edge sub-objects within a solid too like that.

- Michael