I've had students who, when I would speak about the necessity of blending and harmonizing with uke in order to create Aikido, would assume that blending and harmonizing necessarily preclude violence; as though they're somehow mutually exclusive. They're not.
Blending and harmonizing refer to how I will interact with uke primarily with regard to our mutual motion. In blending with uke's motion I seek to avoid points of non-tangential intersection along our mutual paths. Successful blending gives rise to a harmonious relationship between uke and myself as it relates to our motion and connection. The result of this interplay of forces is that uke is taken off balance and guided to the mat where I will either disengage from him or seek to immobilize him via a pin or joint lock. The meeting of uke and the mat can be thought of as violent due to its ofttimes sudden materialization leading to the abrupt cessation of his motion. It is violent, and no less blendy and harmonious for being so.
Pins and joint locks involve pain due mostly to torquing and can also be thought of as violent. However, the violence is controlled in that I can bring uke to the point where further movement on his part will cause pain, and stop. If he ceases moving he will feel nothing while continued resistance on his part will induce a painful sensation at the point of application of the pin or lock. In essence he will be inflicting violence upon himself. His choice and a natural consequence of his aggression.
In Aikido blending and harmonizing are procedural ideas while violence can be seen as a consequence arising not out of nage's intention but as a result of uke's aggressive behavior.
(Original blog post may be found here