I'm interested in BLM but I have no clue how the gauge works

[Stormfur]

So I dabbled with BLM a bit back in ARR and got it to 50... before gauges were a thing. Now I'm thinking about starting it up again but, I have no idea how to read this gauge or what the things mean. Is there an ELI5 guide for BLM?

[Mhaeric]

See here for the official guide to the gauge. You can also see this in game through a button at the bottom left of your Action window.

The gauge has 6 parts to it:

1. Your Astral Fire/Umbral Ice status is indicated by the colour of the crystals in the top left corner of your gauge. The strength of the stance is shown by the number of crystals: 1, 2 or 3. These stances operate the same way aside from minor duration differences as before, so you already know how they work.
2. The duration of your Astral/Umbral stance is indicated by the timer at the top of the gauge.
3. Enochian's status is indicated in the bottom right of the gauge. If the fan of spikes are black then you don't have it active and if they are gold then it is active. You can also consider the Astral/Umbral stance timer at the top of the gauge as your Enochian timer due to the way they are linked. You want to keep Enochian up all of the time in order to be able to use Fire IV and Blizzard IV. You will maintain Enochian as long as you don't drop out of Astral/Umbral stance completely, so you maintain it by switching between the stances or using a high level ice ability to refresh Umbral Stance between fights.
4. Your Foul/Xenoglossy charges are indicated by the one or two diamonds at the bottom of the gauge. If they're hollow/black then you don't have a charge available. If they're pink then you have a charge available, one for each charge. Foul/Xenoglossy are your highest damage AoE and single target abilities respectively that you toss out whenever you have a charge available and won't lose Astral/Umbral stance, or need to move. Most often this lines up well and synergizes well with being used in Umbral Stance since it doesn't have reduced DPS in that stance and works well as a filler spell until you get your mp to full. You earn charges by maintaining Enochian and gain one charge every 30s you maintain it while in combat.
5. The time remaining until you get your next Foul/Xenoglossy charge is indicated by the pink bar to the right of your gauge. It starts filling from the top and once it reaches the diamonds at the bottom you will receive a charge. This only charges up if you have Enochian active.
6. If you have Umbral Heart charges then these will be displayed as ice spikes in the bottom left of the gauge. Three Umbral Hearts are earned by casting Blizzard IV once you unlock the trait and greatly reduce the cost of specific Fire spells thereby extending how long you can stay in Astral Fire stance. Single target fire spells use up a single Umbral Heart charge, and AoE fire spells (read: Flare) use all of them up. You can also gain a single charge when you use Freeze once you have the trait unlocked (useful for AoE rotations) and when you use Umbral Soul which is your Umbral Stance refresher between combat (useful for maintaining Umbral Stance and starting the next combat with full Umbral Heart charges.)

If you're using simple mode for the gauge then the elements are instead displayed as follows:

1. Astral/Umbral stance status is shown by the colour of the first 3 diamonds.
2. Astral/Umbral stance duration is shown by the timer left of the diamonds.
3. Enochian's status is active if there is a sunburst to the left of the timer. There is nothing displayed if it is not active.
4. Foul/Xenoglossy charges are show by black/pink diamonds to the right and slightly above the Astral/Umbral diamonds.
5. Foul/Xenoglossy timer until the next charge is shown by a pink bar just above the Astral/Umbral diamonds.
6. Umbral Heart charges are shown as white diamonds immediately to the right of the Astral/Umbral stance diamonds.

[PyurBlue]

At 50 it only keeps track of Astral Fire and Umbral Ice stacks. Red means you're in fire, Blue means you're in ice and the number of shards is the level of each. You can find an explanation if you open Actions and Traits in game. The FF14 job site also has an explanation at the bottom:

https://na.finalfantasyxiv.com/jobguide/blackmage/

When you get to 56 you get Enochian, if it's on (it should always be on) then the fence on the bottom right of the gauge is bright. If you lose Eno, then this goes dark.

At 60, Blizzard IV gives you Umbral Hearts which show up as icy needs on the bottom left of the gauage. You spend these on fire casts, ideally only Fire IV, and then at level 68+ Flare for AoE.

At 70 a diamond appears on the bottom of gauge. It turns pink when you hold Enochain for 30 seconds, this means you have a stack of Polyglot. You spend Polyglot on Foul.

At 80 you get a second Polyglot stack and you can spend them on Xenoglossy (single target) and Foul (AoE).