The Overwhelming Force KB to KD unique IO can be slotted in Blazing Blast (same with any attack that doesn't accept knockback enhancements). So, that's one problem solved.
However...
Blazing Blast is currently bugged and does much less damage than its listed numbers.
The damage is about the same as Fire Blast. For twice the end cost, and more than twice the recharge. Ouch! ;)
Unless you like the reliable soft control, it's more than safe to drop the power. Fire Blast ends up *superior* in damage, if only because it lets you run a better attack chain. Blaze - Fire Blast - Flares is trivial to attain, while chains with Blazing Blast will have to stuff a second Flares or Fireball in there.
I came into this thread specifically to check if he had taken blazing blast and link to my bug report if he had, but I see someone beat me to it :p.
I am not at a computer where I can check ingame numbers atm, but unless it got scalled differently than a blasters, you shouldnt feel bad about putting fireball in your Single Target chain, as it has better DPA than both blast and flares. Honestly Sentinel's Single target is low enough, and with IOs you can get enough survivabilty on blasters and corrs that I have kind of dropped them until the rework of their inherent that is supposedly in the pipeline hits beta. Basically I came to the exact same conclusion you did, in that xx/time/dark corrs are just kinda better wayyy too much of the time.
The Sents can definitely be made stupid tanky, but you so rarely need more than deff cap (which can be hit on blasters and easily on corrs) that lower damage and especially the lower target caps hurt too much. The target cap limit tends to be what hurts the most in the few situations where the tankyness actuallyof a sent is actually appreciated.
I mean, if you like Bio for the bonus damage and crazy regen, I have a fire/psy/fire blaster that has perma drain psyche (1200% regen at 10 targets), 40% ranged def, 30% melee/AoE def, and 60%+ S/L res. If you go //scorp instead of //fire you lose some res, but you can easily hit def caps for S/L/E/F/C and ~30% NE.
Long Story short, I think sents are real close to being a strong class, but they still need one more little push to really get there. I can deff see why they wanted to start with them under-tuned and push upwards though, as they have the survivability to be pretty broken the other direction quickly if you over shoot a damage buff by even a little.