A script to do mostly the same as i3lock-blur. Tweak `convert` to get the desired result.

lock.sh

#!/bin/env bash

outputs=`swaymsg -t get_outputs | grep -oE "DP-[0-9]"`

for o in $outputs
do
        swaygrab -o $o ${o}.png
        # Actually, this doesn't work if your monitors arent this size... should be tweaked too
        convert ${o}.png -scale 240x135 ${o}.png
        convert ${o}.png -scale 1920x1080 ${o}.png
        images="${images} -i ${o}:${o}.png"
        cleanup="${cleanup} ${o}.png"
done

swaylock -u $images
rm $cleanup

An update that also fetches the right resolution will be coming at some point.