Fire blast vs Dark blast/Dark secondary

[Snarky]

What are some thoughts on the differences?

[biostem]

Dark blast has a lot more utility, and depending on the circumstance, that may be more valuable than the extra DoTs that fire has to offer.  OTOH, the -res from tar patch and the other control effects dark miasma has to offer may enable you to really unleash the extra damage fire has to offer.  That being said, I'm kind of partial to dark, in this specific case, (though ice blast may be another nice option to consider).  

[Maelwys]

Fire = More Damage by way of Bigger Red Numbers.
It boasts high raw Single Target and AoE damage plus access to Aim.
And Scourge loves "Rain of Fire".
Downsides: Nuke needs you to be in Melee and has a long animation time. Powers have no secondary effect (so limited damage proc opportunities in the ST Blasts)

Dark = More Damage by way of Not Faceplanting as Quickly.
It has decent Single Target damage, plus "Life Drain" and lots of -ToHit debuffs.
It gets to slot an extra Damage Proc (from the Cloud Senses set) in each attack.
Downside: Poor AoE damage output other than the nuke (which also needs you to be in Melee and has a long animation time... although at least it inflicts -50% ToHit!)

[Maelwys]

Also FWIW I've been playing around with a Water/ Corr a bit recently.
It's hitting a very nice sweet spot for me somewhere between the AoE capability of Fire Blast, the Procability of Ice Blast and the sustainability of Dark Blast.

Mix of ST and AoE, a Ranged Nuke, a Scourge-friendly "Rain" power in Whirlpool (despite the animation being ground-centric!), a Self-Heal, access to "Aim" (AKA 'Tidal Forces'), a useful secondary effect and opportunities to slot Achilles' Heel procs in multiple places. It's a little lacking in terms of raw high-DPA Single Target damage compared to the likes of Fire Blast; but its powers take 2-4 additional damage procs (slow and -def) and it makes up for it practically everywhere else.

[Uun]

I'm not a fan of the defender/corruptor version of Dark Blast. I know it's the OG, but Dark Pit is hot garbage and Torrent and Tenebrous Tentacles are only useful if you turn them into proc bombs. Blasters and sentinels get much better versions. In the blaster version, Nightfall, Torrent and Dark Pit are replaced with Umbral Torrent, Abyssal Gaze and Aim. Sentinels get the blaster changes, plus Moonbeam (sentinels don't get snipes) and Tenebrous Tentacles are replaced with Antumbral Beam and Dark Obliteration.

[Jacke]

Why not both?  I have a Fire/Dark Corruptor on CB, eventually on HC.  I plan to make a Dark/Dark Toon for every AT.