blending surfaces-III

I'm trapped a situation to start from an existing surface (the real case might be very different from the current simplified example) to build a transitional face.

 

My solution is to create two Curve on Surface's, which can be adjusted to control the resulting surface. But this type of control seems to be not so flexible. I wonder whether a blending command can be used to create the transitional surface, which thus can be controlled in a much more flexible way.

 

Thanks!