Hi Patrick, Hello everyone,
Currently, the blur filter is simply the blur node from Substance 3D Designer. Therefore, this filter has effect on the 2D view (so only the UV islands), and this is what creates the seams. As an artist too, I know how bothering these blur seams can be.
One workaround would be to blur your layer as usual, add on top of it a paint layer, set its blending mode to passthrough and paint with the Smudge blur the seams.

You could also try to make your own blur filter with Substance 3D Designer, or directly buy the 3D blur from the artist Victoria Holland. To be clear, this is a filter made by the community, I never tried it, and I don't know anything about its quality, but the comments seems good. My only advise if you do buy a custom filter: don't overuse it.
From experience, these filter are rarely optimized and can drop down your project performances.
Best regards,