I think it is better to use f4xxRune (MB) with the Dark Curse up because you have time to see whether the overhead was blocked or not and hitconfirm the MB. That way you can save you meter and being at -11 with armour up is safe and won't allow your opponent start anything. By doing the f4xxDemon Skull you commit to spending your meter and it is good to be +6 but I usually keep that for when there is no armour to back me up.
It is also worth noting that you can save meter by not spending it on Rune MB in corner combos to run out the timer on Dark Curse, but by ending you combos in trance, jump back punch, f212xxskull if you have just set up the dark curse. That way your create a standing reset which leaves you at +2 with the armour up. The next combo after the standing reset should then again be ended in airborne 141 as it will run out the timer without the use of meter.
For Black Magic there are corner combos using very low to the ground air skulls which drain almost three bars of opponents meter using only one of yours. I think Gustavnes had some on his youtube.
Edit: here you go - he uses his b2 to start a combo as it was an overhead back then. So you have to start it with f4 and it will do more damage. But what is important is that the draining properties of the specials in Black Magic circle remain the same:
The Final Hex isn't worth the resources to be set up of the Ex trance. I find that the best way to use it is when your opponent doesn't have meter (or when you have drained it before hand with a Black Magic combo) for wake up and you end your corner combo in 141, Final Hex. Here you go for f212xxRune MB on knock down. Again Rune MB does more chip damage then Demon Skull. And when the opponent doesn't have meter to armour through the gaps you are free to delay the MB after the rune to it's maximum so that you are at +7 and can do another string into rune.