If Jhin plays Curtain Call, and it's denied, can Jhin, Meticulous Killer still be played by cycling a blue rune?

Yes, you can.

Jhin, Meticulous Killer checks whether you spent 4 or more energy to play a spell this turn, not whether that spell actually resolved. Curtain Call has a base cost of 4 energy — once you pay it and the spell becomes a Finalized Chain Item, the condition is locked in. Defy countering Curtain Call doesn't un-spend that energy. (FAQ #9920, Rules FAQ Clarification 2026-04-29)

So the sequence works: play Curtain Call paying 4 energy → opponent Defies it → Curtain Call is countered → you're still able to play Jhin, Meticulous Killer for [C] by recycling a blue/Calm rune (or any rune of your choice) to pay the power cost.

Two quick distinctions to note: the Jhin Legend (Virtuoso) ability that banishes spells does require the spell to actually resolve — that trigger won't fire if Curtain Call was countered. But Jhin, Meticulous Killer's alternative cost condition is separate, and it only cares about the spending, not the resolution.