Tried one harder - Thought I'd make a vent port like one that would exist on a sport car...

Made a curved mesh surface, then the shape of the port out of one single freeform spline.
It was projected to the surface, trimmed and deleted, leaving the new hole's with it's own native edges.
I copied the projected shape, moved it lower, scaled down, rotated and extruded it.

Then I wanted to blend between the two shapes... Nothing. The shapes had a lot of individual unconnected segments in it,
and Blend only wants to use whole, single curves that reside on surfaces.
I tried to Join these separate pieces into one curve, but since Joining makes it's own separate element, it defeated the purpose and I couldn't use Blend.
I'm not sure where I could've done something different to make two contiguous rings on their surfaces to Blend into a seamless transition.

What I constructed below was done in sections, but there are irregularities in the curvature and even chunks missing.
I take it, that since projecting a curve to a surface does not make a contiguous curve like unto the original shape, then I might not be able to use Blend on these elements.
I can use Network or Sweep to achieve like results, but the Blend tool offers that useful silky-smooth"Tangent" blending ability that is hard to get other ways.