How to make subtract blur mask in Blender Video Editor

I been watching this video where guy shows how to make simple blur mask in UV image editor and VE.

But i need subtract blur mask -just so that the blur will be not inside of the mask but outside. There is Subtract option there but it doesnt do what i need - it just makes whole image darker for some reason. Also i tried Alpha Under - and its not what i want as well.

Invert the mask


Thanks. But i eventually went to do masking in compositor

You can use your masks by inserting a Mask node in the compositor: