In the Blend Bone by Channel's Details panel you can add a Bone Definitions section with (+) Add. After creating a definition, you can define a source bone to pull transform data from, and a Target bone to apply the blend. You can also define what aspects of the transform you want to incorporate in the blend, such as Translation, Rotation and Scale by toggling the available properties. In addition you can define in what Transform Space the transform data is being calculated, such as World Space, Component Space, Parent Bone Space and Bone Space.