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