These two surfaces don’t touch, but I feel you should still be able to create a blend surface between them:
Is that a fully manual operation with COS/ISO creation/extraction and loft + match etc, or is there a tool in Rhino I’m not aware of that does all of this?
Yes, and note my dashed lines in the picture I posted. Are you saying that BlendSrf can work with surfaces and not just edges?
I clicked on two surface edges.
You might need to trim the surfaces first if they are too closed together to forma a nice large blend?
Right, so that falls within this category then:
I was asking whether there was a tool that did not need this.
You can create the blend with history and then afterwards use the edit functiona and drag the surface edges.
How do you expect Rhino to know how smooth the blend should be?
FilletSrf for either a circular arc or G2 blend surface. For a different blend surface use FilletSrf, delete the fillet, then BlendSrf between the trimmed edges.
FilletSrf now has an option to do a ‘blend’ instead, that seems to work…oh that’s what David said.
Perfect, thank you!
(I get a dread sense of deja vu now that I see this answer…) 
There are a few ways to do that. Personally, I prefer this one. because it’s gives me the best control over the shape and position of the blend edges, as well as the least amount of control points:
You may also try manuallt building some simple 4-sided surface with 4 control points total whose two edges are relatively close to the target surfaces. Then, rebuild the surface to degree 5 with 6 control points and use “Match surface” with the “CurveNearSurface=On” option. That will let you match the surface edges directly on the target surface rather than its edge.
