Let's say we're playing Tarn Mission/R7 Astromech; player has a TL on opponent flying a ship that has predator equipped.

Opponent attacks. and rolls, say, two hits and a blank. Does Tarn's R7 kick in before Predator?

Yes - defender may modify the attackers attack dice before the attacker gets his opportunity to do so.

Note also that the dice you force the attacker to reroll may not be rerolled again

Steps outlined here: http://xwing-miniatures.wikia.com/wiki/Phases_/_Order_of_Gameplay