Is there any way to hide part of polygon boundary symbology if they overlap with my area of interest - other polygon?
+ I can't change the symbology to hide under
+ I can't change the shape of red poligon outside the black one to mask it
ArcGIS Pro 2.3
You might be able to "fake it."
1. Add another solid stroke symbol layer.
2. Set it's color to match your background.
3. Work with the dash pattern so that it just covers the red.
I'm sure you'll have to play around with dash patterns, but hopefully something like this could work for you...