Tarek reacted to MEB in How to apply a blur effect to picture under pen shape (poly art)
Welcome to Affinity Forums :)
Go to the Layers panel, select the image layer then go to menu Layer ▸ New Live Filter Layer and select the Blur filter you want from the top section. This will apply the filter just to the image leaving the grouped triangles above intact (the filter will be attached to the image layer).
Tarek reacted to R C-R in How to apply a blur effect to picture under pen shape (poly art)
I am not sure I understand what you are trying to do but if you want the pen strokes (which I assume is what you mean by border) to remain unaffected I think you will need to create a second group of unfilled triangles with the Pen tool (or with the Triangle shape tool) & place that group above everything you want the blur to affect.