From:  Michael Gibson
2232.2 In reply to 2232.1 
Hi Pilou - Blend is always oriented towards a surface. When the edge that you pick has 2 surfaces connected to it, it is kind of random which one will be chosen.

To get more control over that situation, delete one face of each box so that the edges you want to blend are only attached to 1 surface and not to 2, then you have a reliable way of knowing which surface the blend will be using.

- Michael