In essence, nothing changes.
Since you say 70-80 , I assume you know how to play a lv 70 BLM

You get a single target flare that you use as your final skill before going in ice mode. (Usually around 800~1200 mp). And that's your very first new spell so you have 8 level to practice using it !

Umbral soul is just a replacement for transpose spam in dungeon or during long boss transition. It just allows you to stay in ice mode and build up umbral heart . You are NOT suppose to use that in your main rotation. It a transition/between pull tool (QoL basically )

Xenoglossy is a single target, instant Foul. So if you know how Foul works, it is the same... But instant

So technically , you get 1 new spell to add to your rotation , Despair (single target flare). The rest is exactly the same as your old kv70 rotation, just with some minor tweaks. Enjoy your instant Foul (lv80 Xenoglossy) ! It's really fun