Hi Pilou - Blend actually is easier to use on a open surface rather than on a closed volume.

It actually works on any edge, but with a closed volume an edge is shared to belong to 2 surfaces, not just one surface. So when blending with an edge of a closed volume, it is not predictable which of those 2 surfaces will be the one used for the alignment.

When you have an open surface with an edge that is not joined between 2 surfaces, then that edge belongs to only a single surface so that makes it easier to understand which surface the blend will align with since there is only one to choose from.

Other than that, one problem that is easy to run into with Blend is if you have generator curves overlapping exactly on top of the surface edges - if you want to do an edge/edge blend you need to select the edge and not the generator curve so it is good to hide or delete the generator curves in this case so you can expose and more easily pick the edge rather than the generator curve that is positioned exactly on top of it.

- Michael