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Spells as written require line of sight so if the character cannot be seen I wouldn't even allow "blind spellcasting" with darkness penalty. Perception test with penalty and then spellcasting without penalty. You spotted your target, astral arc is formed so nothing interfere with spellcasting (except cover which increases MD of a target).

Ha, don't you just love metaphysics. So many possibly valid interpretations, and I actually forgot about some of that (so I'll probably have to think some more on this). So we have a few different scenarios:

-Caster inside and affected by Ethereal Darkness tries to spellcast on a target outside.
-Caster outside area of effect wants to spellcast at Target inside area of effect.
(These should be resolved the same, and take into account the line-of-sight "or otherwise sure of target's location" restriction - which I usually interpret as Touch, but sensing their Astral Imprint counts too, and players get creative... which leads to:)
-Caster in or outside Ethereal Darkness wants to Astral Sense something's imprint (not pattern) in order to be sure of the target's location.

And finally, keep in mind that the effects of the spell are localized to "physical space" (unless both the original caster and target are in astral space, in which case the spell doesn't even manifest in physical space, all the effects manifest in Astral Space). On that note, most descriptions of astral space I've read include the fact that it is "dimly lit" by the magical pattern of the Living Earth (I hope I'm remembering that correctly), but the Darkness spell would not be manifest in astral space AS Darkness unless both original caster and target are in Astral Space.
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I think Baravakar has put forth a good example (that avoids the "everyone's in astral space" scenario for simplicity), and most of his conclusions follow my line of thought, especially the point that Astral Sight/Sensing will allow them to side-step the Darkness like the other *sight abilities. The only thing I'm not sure on is that last bit, does it sensing an astral imprint given enough information to target a spell (beat MD of astral space in the area), or does it take reading their Pattern (beat their MD)?

Based on what Mataxes mentioned earlier, it just takes sensing their Imprint (other spells/abilities require a Pattern lock).