Sounds reasonable to me.
Updated with changes (cost now 5 :darknes and 4 , down from 8 and 7 )
Do the costs on the other seals made so far look ok?
(Dragon Seal of Freedom and Dragon Seal of Resilience)
At first I thought the Freedom one was expensive, but then I compared with your Wind Moth and I thought it compared well (excluding the cost of the dragon). I'm not sure about the resilience one. Maybe the fact that you need a dragon and that they are expensive already would warrant a reduction. (going back to your moths, you can play them all early in the game).
The aether seal will probably need to be cheaper, to account for the need of a duo to use it
I can always allow the aether one to target immaterial creatures.
Actually, I've been debating allowing it for all dragon seals, since doing so would require either using the (very expensive) phase dragon, or using an ability to make a dragon immaterial...
I'm not sure if these would start to get OP that way though.
The defensive ones would probably be fine, but when I make the offense seals (probably fire and water, maybe aether and gravity as well), things could get ugly.