Skulk matters only as blockers are chosen. Modifying either creature’s power after blockers are chosen won’t cause the attacking creature to become unblocked.

If you cause a creature to have 0 power or less, use the actual value (which may be negative) to determine whether it can block or be blocked. A creature with skulk and 0 or less power most likely won’t be blocked, but it won’t deal combat damage and won’t trigger any abilities that trigger when combat damage is dealt.

For more information on double-faced cards, see the Shadows over Innistrad mechanics article (http://magic.wizards.com/en/articles/archive/feature/shadows-over-innistrad-mechanics).