Hi Simba, currently your shape there is a closed solid - but Blend likes to work off of unattached open edges so it knows for sure which surface it should blend to. When edges are joined between 2 surfaces it is ambiguous which of the 2 possible surfaces it will target.

So use Edit > Separate or delete the bottom end cap on the upper piece and delete the interior walls on the bottom piece and then you'll be able to do a blend as in the attached file.

But you will not get very good shaping at the front part since it is so elongated and also goes through a sudden change in shape in these areas here:




You may need to do the blend only up to this far on the bottom edges and then use some other means for filling in the remaining area:




- Michael