Hi Pilou, Blend is meant to create a smooth surface - an object with a perfect 90 degree angle in it will have a corner in it and will not be smooth, so it won't work as a Blend.

Normally to put in something like that you would just draw a couple of polylines and then use Construct/Planar to build a planar surface from the outline.

If you like you can also select 2 edges at a time and use Edit/Extend to make extended lines where they intersect one another, like this:





- Michael