Hi,
I think flip of a curve reverses the start and end order.

Please post a .3dm.

- Brian

Hide your curves, before selecting the two edges to blend.
The curve overlying an edge is not the same as the edge.

Apparently your first picture shows a curve connecting an end of the upper curve to an end of the lower curve, which is a blend?
(The blend is another curve, not a surface.)
The curve blend has a history, so after the curve blend, flip of one curve will cause the opposite endpoint of that curve to be blended instead.
(The first curve blend jumps to the other end.)