By default when blending from one clip to another, the system uses the transition properties you edit in the animation or procedural clip you blend towards. Those are not what we call mannequin transitions though, those are specific descriptions of how to transition from one fragment to another.
(the default blend time when blending to a clip is edited in the clip itself. See the Mannequin Animation Clip Properties and Mannequin Fragment Editor)
To create specific transitions use the Mannequin Transition Browser and Mannequin Transition Editor.
See Mannequin Editor Tutorial 3 - Transitions.
Transitions are stored in the animation database files alongside the fragments.