Are black dragons immuned to ball of lightning ?
They never seem to get targeted

No they are not, not entirely. Since they have 80% magic resistance (ball of lightning does magic damage) they only receive 1/5 of the original damage. So instead of loosing 20-50% of their numbers, they loose only 4-10% of their numbers. Ball of Lightning targets the unit in range that it does most damage to, that is why black dragons are seldom targeted.

Astral damage is the way to go against black dragons. Spells like Death Star and Trap works nicely. Fiery Phantoms are the best pet ability against black dragons.

I like lighting spell.
I think It's useful against armies with one or two strong numbers units, because it concentrates damage against those units (with a great overall damage total) and also softers them before the beginig of the unit vs unit combat.
Also, its cheap in rage cost and is the only dragon spell that generates rage (on every strike), so its also a way to boost your rage.