Hang on. Isn't that the point of uke? Shouldn't uke be playing the role of an attacker in order to allow nage to practice their technique in a simulated attack scenario? If uke was all about blending and being 'aiki' all the time, then how would nage be able to practice effectively?
I don't think of nage's job as being "blending".
I think of both nage and uke's roles as being to make an effective center to center connection:
uke in order to make an effective attack which means an attack on nage's structural integrity either via a grab, blow, strike, etc - nage in order to effect uke's structural integrity to effect a pin, roll, fall, etc.