“Apply a blur using a Live Filter layer, invert the live filter layer (which makes its embedded mask completely black), then paint the blur effect back in where it’s wanted. This is completely non-destructive, can be made as strong or as weak as one likes, can be turned on and off at will, can be modified with opacity and/or blend modes, and (best of all) it’s so damned easy!” – smadell, Affinity forum advanced member
Watch video instructions on Affinity forum: https://forum.affinity.serif.com/index.php?/topic/105047-blurring-a-part-of-a-picture-such-as-the-face/&tab=comments#comment-567242
Here’s an example of faces that I blurred out by following the Affinity forum’s instructions. Worked out great!