Hi Andre, just to illustrate what Burr wrote - these are the faces to delete:



Then you can select 2 edges like this and run Construct > Blend to make a blend surface:


The reason to delete the faces is to control which surface the blend will be smooth to. When it is a joined edge shared between 2 surfaces it doesn't know which of these possible directions to use:





You want the face to be removed so the edge only belongs to one face.

- Michael