Holy macaroni - and I say this more out of surprise than anything, not giving you hard time for asking, but I think that this is the third time that I've seen this question pop up in the past week - and before that I've hardly seen it asked before.
No, unless your amp has a blend function already built into the FX loop, plugging something into the return usually bypasses the amp's pre. Typically the use-case for FX loops is to take the amp's preamp output via the send, run that through FX (that work line-level), and then return back via the... return.
In that case you'd have to use an outboard blender or mixer in the loop to do the kind of blending you're looking at, though it is possible. I toyed with doing something similar for a little while, but decided against it in favor of simplicity (it wasn't worth it IMO).