why does "blend" not work here?

 From:  Marc (TELLIER)
4993.5 In reply to 4993.4 
Nice solution Bemfarmer!

Just make sure that you are using the edges of the surface and not the 2d curves. (select surface and then the edges as a sub-selection)
Then hit tab and type: merge, the edges will join themselves.

After merging them you can perform the blend between the two surfaces.

It's easier to hide or delete the overlapping 2d curves in your drawing prior to this operation.