I found myself investigating these fields today, they are actually both used.
BlendOutTriggerTime >= 0: The anim montage will start blending out early before montage has finished playing based on the BlendOutTriggerTime. But it will then blend for the next BlendOutTime seconds, which can go even beyond the duration of the montage. You can have a BlendOutTriggerTime of 0.0, and a BlendOutTime of 0.1s, and it will continue blending the last frame of the montage for 0.1s after it’s finished playing.
BlendOutTriggerTime < 0: In this case BlendOutTriggerTime is ignored. The montage starts blending out BlendOutTime seconds before the montage ends. However it cannot blend beyond the end of the montage.
Honestly, I find the behavior of these two fields really confusing, but I imagine changing them now would break a lot of projects out there.