I guess it would pop up a dialog, to select the blend ratio, as a prompt perhaps, right after the material group picker dialog is ok'd?

This could save a lot of copying and pasting, no?

I appreciate the logistics of this idea - and the array of user choices implied by it - may override it feasibility...

...what about if the add / blend material button simply loaded in the material files nodes... but didn't connect then up to the existing nodes? That would save the copy / paste exertions at least? You'd then just need to rewire as you saw fit... (which makes it more of an advanced user feature, sure)