if you play astral heron for your first card to a battlefield. can its card cost reduction be used for hiding a card?

No. Astral Heron's reduction only applies to playing a card, and hiding is not playing a card.

Hiding is a separate Discretionary Action — Rule 811.1.c.1 explicitly states "Hide is not a subset of Play," and hiding doesn't even open a chain. Astral Heron's discount applies to "your next card" during the cost-determination steps of playing a card (Rule 356); the [A] you pay to hide is a cost of the Hide action, not a card cost, so no card cost reduction touches it.

It also wouldn't matter if it could: playing a card from Hidden costs [0] (base costs ignored), so there'd be nothing to reduce. The only card-play cost Astral Heron would meaningfully discount is a card played normally from hand.