Blessed or Geomancer

For an Obby Tank?

I was wondering on the Geomancer Insignia's does the +10 vs ele, and +10 vs earth Stack? If not... then wouldn't it be best just to use Blessed Insignias?

ok obby tanks require geomancer - reason is you end up with skills being able to convert all dmg to earth damage which is reduced with your geomancer armor along with mantra of earth which gives energy when taking earth damage.And i believe the armor may or may not stack as if using skills all damage via stone striker becomes earth damage - someone may need to verify.
Blessed may work but could be less effective - eg vaettires deal earth damage so instead of needing stone striker you have a free skill slot.
At a guess all if not most terratanks ( obby tanks as you call them ) use geomancer as a rule ( most i know do ).

Alright I was wondering because I see the E/A Against the charr uses either Survivor or Blessed Insignias.

Blessed>geomancer.
If you obby tanking you're gunna have an enchant, so you have armour for that. However if you're taking mantra+stone striker it really makes no difference

to answer your question, it does stack. blessed gives +10al armor vs physical and elemental dmg, so for general purpose its better. but if you plan to use stone striker, than geomancer would gives you +20al vs earth dmg, being better.

If you use Stone Striker or are farming mobs that deal earth dmg (e.g. Vaettir) then Geomancer is the better choice. Personally I have multiple armor sets, one of them outfitted with geomancer insignia. But this can be costly if you don't use/need it. Anyway, you can farm Vaettir with just blessed insignia and obby+stoneflesh alone so geomancer's isn't really needed for that either.

And yes the two geomancer subtypes stack: if you are getting elemental damage you have +10 armor, if you are getting earth damage you have +20 armor (+10 vs elemental and +10 vs earth).