I have to say, Kismet is offering good advice here.
I'm nowhere NEAR as familiar with the Dark Armor powerset, so grab the salt shaker for the following musings on my part.
Since I have no way to parse Mids' code hashes into builds (since Mids' won't work on my computer), I don't know if you're doing this already, but ... it occurs to me that there's an opportunity in Death Shroud. 6-slot Death Shroud with Overwhelming Force (to get the -4 Knock Protection set bonus). Unless I'm misremembering, Dark Armor has no "native" Knockback Protection, so you'll need to slot for it (somewhere). However, the REAL advantage of putting Overwhelming Force into Death Shroud (and many other PBAoE damage auras) is that because the proc is a flat 20% check every time a Foe is successfully hit, (I think) you shouldn't be looking at a 10 second suppression of proc chances after a successful proc when putting the Overwhelming Force proc into Death Shroud. In other words, I suspect that you'll be getting a 20% Knockdown chance every 2 seconds against anything within your 8ft PBAoE damage aura ... which isn't bad.
As for slotting Resistance Toggles (Dark Armor has 3) ... one option would be 2 slot Titanium Coating (+1.5% Resist E/N, +2.5% Resist Mez set bonus) plus 3 slot Unbreakable Guard, including the +7.5% Max Health in one of them (2.5% Endurance Discount, +2.25% Resist E/N, +3.75% Resist Mez set bonus). This would still give you 1 slot to play with in each toggle, for which I would recommend Steadfast Protection: Resist/+3% Defense, Impervious Skin: 7.5% Resist Mez/+25% Regeneration, Gladiator's Armor: Resist TP/+3% Defense. If you 6-slot Death Shroud with Overwhelming Force, you won't need to spend a slot in your Resistance Toggles on Knockback Protection. Additionally, with the 2-Titanium Coating and 3-Unbreakable Guard combo done 3x you'll add +11.25% Resist E/N, you ought to be able to get at least +50% Resist enhancement into Murky Cloud, which will give you at least 30% Resistance E/N as your foundation.
So with 4 powers with +5 slots each, you're already over 40% Resistance Energy, which would not be bad for a Dark Armor build. You could then use additional set bonuses from other powers to increase that amount even higher ... and that's before including (Superior) Might of the Tanker procs for +Resistance into the mix.
So while Kismet is quite correct on the point about Resist Energy being something of a "weak point" for Dark Armor, you still have OPTIONS to be able to shore up that weakness through clever design of your build via set bonuses. You probably won't be able to reach the Resist Energy hardcap of 90% on a Tanker ... but then, do you really NEED to get your Resist Energy to 90%? Is this a case of "go max or go home" (so to speak)?
Sure, it would be NICE to be able to get to 90% Resist S/L, E/N, F/C and T/P (if practical) ... but is it always absolutely necessary (or GTFO)?
I can envision build strategies that work towards getting as much Resist as is practical for you, while also diversifying into +Max Health and +Regeneration ... that way, whatever cannot defeat you
only makes you colder and more embittered.
Your mileage may vary, of course.