Blending, in it's most general form, is a process of mixing. In the context of Aikido, blending refers to the mixing of uke's and nage's individual motions so as to bring their conflicting trajectories into congruence in order to facilitate the execution of a technique.
The language just doesn't work for me. I don't consider my harmony or any of the definitions of congruence, mathematical or otherwise, (except maybe compatibility) to describe well what is going on when I receive uke's attacking energy into my physical system and use it to affect their structure.
Compatibility is closest because I make their structure dependent upon mine, but I don't still don't feel that I'm blending my energy with theirs. I'm letting it move through me and adding a tweak here and there to change it's trajectory or rebound upon them. It's definitely not what I consider blending.
But maybe I am just worrying too much about semantics here, although at especially beginning levels of training I find sematic descriptions incredibly important in the responses they illicit from untrained students.