Some interesting numbers on 4.0 BLM

[Alkimi]

Make Blizzard IV reduce magic resistance by 5%. Forces you to use the "proper" rotation, slightly improves BLM dps and also provides raid utility.
or
Make Blizzard IV reduce fire resistance by 10%. Brings BLM closer to SAM dps but no raid utility (except I guess Katon and a couple of RDM spells)

[Galvuu]

Make Blizzard IV reduce magic resistance by 5%. Forces you to use the "proper" rotation, slightly improves BLM dps and also provides raid utility.
or
Make Blizzard IV reduce fire resistance by 10%. Brings BLM closer to SAM dps but no raid utility (except I guess Katon and a couple of RDM spells)

And it's good utility!
This is a very good idea

[Ali5553]

Make Blizzard IV reduce magic resistance by 5%. Forces you to use the "proper" rotation, slightly improves BLM dps and also provides raid utility.
or
Make Blizzard IV reduce fire resistance by 10%. Brings BLM closer to SAM dps but no raid utility (except I guess Katon and a couple of RDM spells)

tbh i dont realy like this idea. simply from a logical point of view (i know its called final fantasy but even fantasy follows some kind of logic . Umbral hearts are like a gameplay mechanic belonging to a BLM so i dont see how it can be equated to a debuff.

i suggest that they should increase the potency of the spell cast within umbral hearts. each umbral heart increases the spell potency cast in AF by 30 so for example:
if you have 3 umbral hearts up and you cast 1 fire spell in AF than the first spell will get a 90 potency boost, the second spell will get a 60 potency boost because one of the UH's have been used and the third spell will increase its potency by 30.
I did some quick calculations with this using only the opener rotation and it showed an increase of about ~ 5%.

They did a pretty good job on BLM, they only need to tweak it a bit in terms of damage and its fine. by giving umbral hearts more reason to be used outside the opener, within ley and aoe.

[Waliel]

math

B3 and F3 is a 3.5 cast and why would you cast B3 anyway? Their damage also can't get halved.

10 mobs with Thunder:

T4 B4 F3 F2 F2 Flare Flare - 443,6 PPS
(2300+260+216+1440*2+2340*2)/23,3

T4 B4 F3 Flare Flare - 397,4 PPS
(2000+260+216+2340*2)/17,3

T4 B4 F3 F2 F2 F2 F2 Flare - 435,1 PPS
(2300+260+216+1440*5+2340)/28,3

Bonus:
T4 F3 F2 F2 Flare - 450,6 PPS
(2000+216+1440*2+2340)/16,5

You need 3 ticks for normal T4 to be roughly equal to F2 (140 and 144) and use TC pretty much at will. Foul would be extra 4225p every 30 seconds.

Did a quick test and you have to wait a tiny bit to get enough mana for 5xF2, and even longer come next patch because of shorter B4 (already included above).

[Kabooa]

B3 and F3 is a 3.5 cast and why would you cast B3 anyway? Their damage also can't get halved.

I was admittedly tired. Transpose F3 will come out to about -1.5 on all the scenarios listed then, though the MP tick waiting means F2 will likely not beat out the F2/Flare set up.