To be able to use it on a mask, you have to set your input image name as "input", and your output image as "output".
Here the blurHQ filter from Designer, modified to be used in Painter.
Sadly Painter has a big limit: there is no uv padding, so applying a blur will result in evident seams.
Also, Blur effects are so SLOW... I wonder if there will be made optimizations to substances to make them faster.