If you Hidden Blade a unit, can you activate Sun Disk after Hidden Blade but before reviving an Immortal Phoenix from the trash?

No, you cannot activate Sun Disc in that window.

According to the RiftJudge FAQ (ID 2075), Sun Disc is not a reaction and cannot be activated after a spell has been played and is in the process of resolving (or triggering its effects). You must activate Sun Disc before playing the spell that triggers the Phoenix.

The Reasoning:

1. Timing Restrictions: Sun Disc has an activated ability. Activated abilities of gear/units are limited to your Main Phase during an Open State (Rule 145.2). When Hidden Blade is played, the game state becomes Closed (a chain is created). You cannot activate Sun Disc while the chain exists or while Hidden Blade is resolving.
2. Triggering Immortal Phoenix: As established in RiftJudge FAQ (ID 2182) and Rule 376.2.c.1, Immortal Phoenix's triggered ability only enters the chain after Hidden Blade has fully resolved and left the chain.
3. The Window: There is no point after Hidden Blade starts resolving where you regain the Open State necessary to activate Sun Disc before the game checks for and places the Immortal Phoenix trigger on the chain.

Correct Sequence for Sun Disc:

To use Sun Disc to make a unit enter ready, you must follow this order during your Main Phase:

1. Play a different card: You must finalize another card first to satisfy the Legion requirement (FAQ #9805).
2. Activate Sun Disc: Exhaust Sun Disc while the state is Open.
3. Play your spell/unit: Now that the effect is active, proceed with your turn. If you play Hidden Blade to kill an enemy unit, and your Immortal Phoenix is in the trash, its trigger will be handled after the spell resolves.