Assume only blend with two pose.
It seems that the Blend Depth property decides two things:
1 whether it’s positive or negative decides to use the second pose or not.
2 the number decides how long it takes along the skeleton tree to completely use the target pose, which is decided in note 1.
eg:
number 3 means – the skeleton node’s grandchildren and those below will completely use the Blend Pose 0, but the node itself and it’s children will use blend weight of 1/3 and 2/3…
number -2 means – the skeleton node’s children will use the base pose
check the function “FAnimationRuntime::CreateMaskWeights”