Filters menu (to access the custom blur filter in Affinity Photo)
Blur
Custom blur filter command
Set values in table such as entering 1 or -1 or 2 etc
Set divisor if available
Set offset
Apply
The custom blur filter in Affinity Photo can be found in the blur category of Affinity Photo's filter menu, it is a powerful destructive filter effect and can be used with images as well as combined with other resources