In this case you are trying to blend two faces - not a solid.
Modeling Express SPecific - If you create a solid (Extrude an rectangle) you easily can add a blend.
In the case shown the blend should be applied to 2 faces - not generating a solid part.
The professional version of Modeling and its Surfacing Module it allows you to create a Face Round on such connected faces. Modeling Express does not allow you to do this. Here you need to create a solid first.
