If you click on your Dark Treasure in the backpack, you can see which items can pop out of it.
About the items in it:
Equipment, or cosmetic items, are items that can be equipped in the loadout menu of the customize tab. They have no effect on gameplay, and exist only to alter the look of the heroes and their abilities. Equipment can be earned in a multitude of ways: they can be randomly found after Matchmaking and Co-op matches, by leveling up through Battle Points, by opening Chests, or by buying them in the Dota 2 Store. There is also a small chance to receive an item of higher rarity by deleting items (not applicable to chests).
source: Dota 2 wiki
About the keys:
Keys cannot be found from random drops. Upon purchase, they are immediately tradable, provided the user's first Steam purchase was more than 30 days ago
So, in the end: items are just cosmetic, you can still trade them upon opening the box. You require to buy the correct key from the shop. Some chests may contain a special kind of courier.