If I Defy Hidden Blade, the Defy gets countered by Riposte, can I then Not So Fast the Hidden Blade after Riposte resolves?

Yes — once the chain works back down to Hidden Blade, you get a fresh window and can counter it with Not So Fast.

Order of events: Hidden Blade → your Defy → their Riposte. Riposte resolves first and clears Defy, which then does nothing (Rule 426.1.a). After any item resolves, the chain returns to Step 2 Execute rather than rolling straight into the next item, so nothing resolves underneath you (Rule 340.4).

One priority caveat: after Riposte resolves, Hidden Blade's controller controls the newest remaining item, so they get priority first — you act once they pass (or after whatever they add resolves). Not So Fast is legal on Hidden Blade because it's an enemy spell that chooses your unit, and its cost or Hidden play doesn't matter since Not So Fast has no cost limit.