This seems like a shape where it would be easier to construct it as a smooth thing doing it all at once in one surfacing operation, like making a single loft or sweep for the entire shape in one go.
But trying to connect the pieces - there is a significant enough transition between the 2 parts there that just a single blend like you've done is not good, it swoops around too much and then since the edge has such a dramatic curve, it is hard to construct something else off of it.
In a case like this, you need to create some additional curve structure, so that you can create some surfaces in between the shapes so that you have more control over the transition.
Here is one approach to doing this:
I started by doing a separate on everything to make it into individual surfaces, and then deleted your old blends. Then I created a new curve that was kind of a half-way transition between the existing top 2 edges. I dragged out a construction line between the endpoints of those edges and placed the endpoint of the new curve half-way between them using the mid snap on the construction line. Then I duplicated the edge curves with ctrl+drag and scaled them slightly and positioned the results slightly inside. This results in the figure all the way to the left.
Then I created some guide rails to help define the inbetween shape. To do this I used Draw curve/Freeform/Through points. I went through 5 points there to make the shape. This results in the 2nd figure.
Then I trimmed back those guide rails. I wanted to create them longer than needed and then trim them back, so that the curve tangent at the endpoints would be pointing in a favorable direction towards the existing surfaces. If the curve directions at the end are pointing off in some sharp direction it will make things difficult because the surface you construct there will also follow that. Trimming back results in the 3rd figure.
Now there are enough curves to create the in-between surfaces, I used 2-rail sweep to construct each of those.
Then for the final figure I used blend to create surfaces along the top and bottom, and then used 2-rail sweep to fill in the remaining holes.
The result is still kind of lumpy - I didn't really spend any time on this, to make it smoother you'll want to spend some more effort controlling the shape of the in-between piece, probably with some more profiles for the sweep. If you look at the in-between section from the side you can see that it is kind of slightly "S" shaped which is not so good. Instead you want to put some more profiles in there to make it more evenly shaped and more subtly curved.
Eventually there will be more controls and some additional surfacing tools that will make it somewhat easier to do some of this stuff. But even when those are available, you'll still need to do a similar procedure as this when you have a significant transition between shapes - you just sort of need to draw more curves and give more of a definition to the type of shape you want in between there.
I hope this gives you some ideas on how to approach this.
- Michael
