Black Mage : A few questions

[Jeeqbit]

1- First of all, I regularly see rotations that start with ice magic. In my head, ice magic is good for gaining mana. Since at the start of a rotation I still haven't used any mana, why start a rotation with ice?

At level 80, casting Blizzard IV gives you 3 ice stacks. Each of these stacks allow you to cast a fire spell for no MP. So once you switch back to Astral Fire, you get 3 Fire IV for free and the rest costs MP, but before you cast the ones that cost MP you usually need to refresh Astral Fire with a Fire I cast. That moment spent in Umbral Ice doesn't just restore your mana but also allows you to be in Astral Fire for longer.

2- I don't understand the Umbral soul spell at all. (level 76) I may well reread the spell, but I don't see the point. Can someone explain it to me?

In my experience it's most useful for AoE. After you have got off your flares, going back to Umbral Ice and using that gives you ice stacks which will allow you to do extra flares by mitigating the MP cost. It's frequently used for maintaining Umbral Ice between pulls as well.

3- Some spells use all mana. I imagine the goal when using one of these spells is to use manafont afterwards?

If it uses all mana, you are supposed to use it when mana is low but not depleted as a finisher before switching. But you should read the tooltip because it may only be intended for attacking multiple enemies and not as part of your single target rotation.

Despair is used only when the mana is low and you have got out all of the Fire IVs that you can, before switching to Umbral Ice.