Suppose I have a Saryth, the Viper's Fang, a Hooded Blightfang, and a third creature without deathtouch or vigilance. I attack with that creature, causing it to tap so that it now has deathtouch from Saryth. Does Hooded Blightfang trigger?
My analysis is no, the Blightfang will not trigger, because of rule 508.2a:
Abilities that trigger on a creature attacking trigger only at the point the creature is declared as an attacker. They will not trigger if a creature attacks and then that creature’s characteristics change to match the ability’s trigger condition.
I interpret this as "abilities triggered on attacking trigger only during the performance of rule 508.1a, declaring which creatures will attack, and not during 508.1f which taps the creatures". Am I correct?