Hi mir4ea, one thing that may make your blend case there easier is to select all those edges that you wish to blend and then run the Merge command on them:
http://moi3d.com/2.0/docs/moi_command_reference10.htm#merge

That will merge together those fragmented edges into just one single long edge on each side which will be easier to blend.


Also on a shape like this where you have a protrusion the same size as a hole, it's probably better to round off the edges using fillet rather than blend - because Blend does not trim away any area when you have the same size hole as the object you are blending, it will force the blend to swoop around and make a rather bulgy result. If you instead do an extrusion and then fillet that, you'll get a less bulgy type of rounding.

So the steps you may want to use are like this, start with your surface and your 2D cutting profile:



Select the surface and run Edit > Trim and then use the curve as the cutting object. At the stage in Trim where it asks you to pick which pieces to discard, just right-click or push done to keep all the cut up pieces without discarding any. That will then give you an outside surface piece and an inside surface piece like this:



Select the inside piece and punch it out into a solid by using Construct > Extrude. You'll then have this result:



Go around to the back side of the solid and select the bottom face of it here:



And then delete it so the protrusion is open on the bottom like this:



Now join together those 2 pieces so you'll have one connected piece and now you can use Fillet to round those edges:



Hope this helps! Usually you should be trying to build more pieces of your model as solids using things like extrusions as part of the process.

- Michael