ToME: the Tales of Maj'Eyal

If as a shadowblade I get a seer's quest and take the talent category reward of spell/divination, will my mastery be 0.7 or 1?

I don't think it will be an option at all for shadowblades. Escort rewards, if I'm not mistaken, can only give you trees to unlock, not unlock them.

That said, you can still take the talent rewards (Arcane Eye, for example) and they will be at mastery 1.0 since that's where you already have the tree.

I vaguely recall unlocking a tree at 0.7 mastery when I'd had it at 1.0 before, but I believe this was a long time ago, in an ancient beta, and my memory could be deceiving me.