The API function GetCollectibleBlockReason(collectibelId) will return "not blocked" (0 -> COLLECTIBLE_USAGE_BLOCK_REASON_NOT_BLOCKED) if you are using it inside a dungeon or delve with the collectibelId of a mount.
But it should return
Code:
COLLECTIBLE_USAGE_BLOCK_REASON_BLOCKED_BY_ZONE
or
Code:
COLLECTIBLE_USAGE_BLOCK_REASON_BLOCKED_BY_SUBZONE
if you are alive.