Dark Blast is pretty lopsided on a Blaster. I do not feel it pairs well with Dark, though I have run a Dark Dark Soul Blaster for over 1000 badges lol
Now that I am digging into what makes a Corruptor tick, I feel Dark Blast fits perfectly there. Dark Dark is a beautiful pairing. Although understanding it is key. No buildup? Drop Tar patch before the Nuke and knock 30% off damage resistance.... Although late game Dark^3 Corr gets a larger AoE Soul Drain, so.... quite nice. Its really a matter of learning how you are disassembling mobs rather than just raw damage. Dark Blast works better with that due to its slightly odd DoT, to hit debuffs, and immob, KD and other "petty" gambits that all add up to checkmate. Now in a raw damage +4/x8 ITF where the mobs are melting to incarnate nukes this means little, but in a tough fight it sure turns the tides.