Good advice was given above and i will just add that AF is 80% damage while Umbral ice be -10%, so just keep that in mind once you hit lvl 50
Same goes for once you get thundercloud and Firestarter, if both come up at the same time it is best to use thundercloud first as you have chances of it to proc again, i have had 3 thunderclouds in a row a few times :P
Raging and Quelling Strikes are both a must for cross class abilities in my opinion for blm as we can easily rip agro of of tanks.
Ice 3 once you get it essentially replaces transpose, and damage wise if Firestarter procs while you be in umbral ice, best to transpose. You don't want to cast fire while in ice state.
Low lvl thm yes basically you are stuck to switch between umbral ice and astral fire using transpose.