As far as I know, it should work by itself - in the sense that if you pair it with Blessing Spell, Reflect is calculated afterward.
You take X damage, Blessing reduces it by idk 30%, so Reflect would affect the final damage (including after Block Chance/DEF etc.) - which, in most cases, it's already too low to count/be noticeable on mobs.
Or at least that's how I remember it works.