In place of the Blendable drop-down, bone transforms have a Transform reference field, where you assign the transform you wish to control. When a transform is picked here, the Position, Rotation and Scale fields underneath will fill out with the transform's current attributes. You can then change these values to reflect how the transform should be positioned in this pose. You can also manipulate the transform in the scene using the second transform Gizmo that appears when a Pose with Bone Transforms is open. You can change which bone the Gizmo is controlling by toggling the checkbox next to the Transform field in the Pose.