hmm No IO's.
I'd take weave and 2 slot combat jumping for defense. That + FFgen will put you at ~32% def. So 13% away from soft cap vs an even con. If both seekers hit even unenhanced you will be at the softcap. If you slot seekers and lead with tentacles you should be more or less softcapped vs +1's, maybe even +2's depending on whether both seekers hit, which if you are facing more than 5 or so enemies they probably won't.
I see you saying "no set IO bonuses" - does that mean no set IO's period? Because IMO, if you slot 3 to hit debuffs without franken-slotting set IO's, you're giving up a rather substantial amount of damage. There's quite a few recipes nobody wants that can be used in ranged attacks (like Ruin, for example), which often can be bought for less than common IO's.
Even with sets, 3-slotted to hit debuffs are going to leave you rather far behind on one of damage, endurance, recharge, or accuracy slotting.
Dark blasts for defenders debuff 9.38%, so 2-slotted, you should see something like 13.5% debuff, vs 14.7% for 3-slotted blasts. A single slot would get you about 11.4% debuff.
Bear in mind that these numbers would be affected by the purple patch, so plus level enemies will feel them less. However, I think 3-slotting blasts is a bit much - I'd probably go with a single to hit debuff; 2 at the absolute most. As always, YMMV, and IMO and such, but even with defender modifiers, I really think I'd want more than just 3 slots for accuracy, damage, recharge and endurance.
I wouldn't put any -tohit in the blasts unless I were frankenslotting. If you're going solo you give up on too much damage/acc/endred/recharge to make reaching the softcap worthwhile. If you're in teams reaching the softcap is kinda pointless since you shouldn't be attacked often enough to make it an important goal. And you'd still be giving up on too much damage/acc/recharge/endred.
If by "no IO bonuses" you mean you're game to frankenslot your powers then it might be worth it. Mixing damage sets, tohit debuff sets and accurate tohit debuff sets will probably get you very respectable amounts of everything your blasts need.
I am currently adventuring with my new traps/dark defender. I am building him defensively, and figure that FFG + Seekers + the -acc from dark blasts will bring me near the defense cap. Can anybody tell me if it will do so and if so if I should swap out some of the -acc I've been putting in my blasts for something else? My idea is to 3 slot every blast that does -acc with that enhancement. But is this overkill? Keep in mind I do NOT use any IO bonuses.