Hi Phil, well those pieces have a pretty big and unevenly shaped gap between them... It's not particularly obvious to me how you'd want the connection to be shaped (EDIT: at first it wasn't but later when I saw how things were aligned from the top I think I understand).
Blend is not going to be feasible there because Blend is focused on making a smooth connection between 2 open edges. In your case here that means the blend wants to swoop off in this direction, so it will be smooth to those flat sides:
But the other edge you're blend with is not really in that direction, it's above it, that means the blend will be forced to try and do a sudden swoopy bank turn in a small area of space. That's going to produce a bad result.
It looks like these shapes are aligned from the top view, so my guess is that you'd want a planar surface along one side. You can use Loft to connect some of the pieces together but you would not want to do just one single loft between 2 complex boundaries that are made up of differing numbers of sub segments and sharp corners in it, to loft something like that you need to do it in strategic pieces, one loft for each side basically instead of trying to do it all at once.
In your case here I'd think you'd want to start with one Loft between these 2 edges:
Then I'd probably delete these 3 side wall pieces because you'll be constructing a larger plane in their place, that sticks downwards:
To fill in the back part you can loft between the 2 edges or also select this top edge and extrude it downwards:
Then for the side walls it looks like those are supposed to be one big plane, so you'd select these edges here and run Construct > Planar to build a planar surface there:
But unfortunately all your pieces in this case are not quite tightly on the same plane enough for Planar to work with them, so to fix that up while all those edges are selected run Edit > Join to glue them into a new closed curve object, select that curve object and go to the Top view and squish it down using "flat" snap (for an example of flat snap see here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=3378.4) and then run Construct > Planar on that flattened curve and hope that the pieces that were off were not too far off enough to be joined in to the planar piece.
Probably a better overall strategy for doing this would have been instead to build the whole upper piece as a sort of wedge-like solid with a side profile like this:
If you constructed that piece as a solid from the start, basically starting with that side profile and extruding it to get the base shape of it, that would make sure all the pieces were lined up. Then you use Construct > Boolean > Union to combine the base solid and the second solid piece together and it will form the edges where they intersect for you. Right now it looks like you're kind of trying to build it one little sub surface at a time rather than working with large solid chunks and using booleans to combine those solids chunks together - things usually go faster and easier when you work with solids and booleans as much as possible and only go down to the individual surface construction level for more difficult situations.
It also might be better to wait to form the hollowed out thickness part of the shape until after you'd got the sort of big protruding things combined first...
Or even without doing booleans and working piece-by-piece you would want this cut here to come from a vertical cutting plane that was the same plane used for the sides of the upper piece:
So you'd kind of want there to be some larger extended pieces like one big extended vertical plane that was common to both the top part and the cutout of the bottom part.
Hope this was what you were trying to do!
- Michae