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).