Hi Rudl - for the blend part you need to select edge objects to do the blend. But in your model you have some separate construction curves that are exactly overlapping over top of the surface edges and those can make it difficult to select edges.

So you need to delete or hide those curve objects so that you can more easily select the surface edges instead.

To do that select these 2 curves:



And then either hide or delete those to get them out of the way.

After you do that it will then be easier to select the surface edges which are in those same locations. To select the edges you will click first on the surface that owns the edge and then do a second click on the edge to switch the selection to it.

Once you have the 2 surface edges selected instead of those 2 separate original generator curves selected, then you should find that Blend will work as you were expecting.

In general it's a good idea to either hide or delete construction curves that are right on top of a generated surface after the surface has been created to get the original curves out of the way.

- Michael