What is the "Black Mushroom" item used for?

In the release version of the game the item "Black Mushroom" is a component in creating the item "Staff of Herding." It is still an Easter Egg referencing Diablo 1 though, but also helps in crafting the item.

The full list of items needed to create the "Staff of Herding" are: Black Mushroom, Wirt's Belt, Liquid Rainbow, Gibbering Gemstone, Leoric's Shinbone, the crafting plans, and a 50,000 gold crafting fee. Additionally, the item can be "leveled up" per difficulty by finding the plans for that difficulty, using a created staff from the weaker difficulty, and paying the crafting cost to create the "leveled up" version of the staff.

List of requirements for the staff and its upgrades:

Staff of Herding: Requires Black Mushroom, Wirt’s Bell, Liquid Rainbow, Gibbering Gemstone, Leoric’s Shinbone, and a 50,000 gold crafting cost

Nightmarish Staff of Herding: Requires the Staff of Herding and a 200,000 gold crafting cost

Hellish Staff of Herding: Requires the Nightmarish Staff of Herding and a 500,000 gold crafting cost

Infernal Staff of Herding: Requires the Hellish Staff of Herding and a 1,000,000 gold crafting cost

The total cost to craft the final version, the Infernal Staff of Herding, is 1,750,000 gold.

It has also been confirmed that the staff is used to get to the secret level.

It's an ingredient for crafting the Staff of Herding, which opens the portal to Whimsyshire, the secret level.

It alludes to a diablo cave quest where Adria asked the hero to find a Black Mushroom item. The D3 item's description comes verbatim from Deckard Cain's NPC response during that quest:

Finding the Black Mushroom in the Cathedral:

Thanks to extensive research by agent86, the Black Mushroom is used to make the Staff of Herding, which is used to access Wimsyshire, a land of rainbows.

Rather than reproduce the whole thing with pictures and videos, see answer here: https://gaming.stackexchange.com/a/67096/16350

To quote from his answer:

Unlocking Wimsyshire requires a Staff of Herding, which can be crafted with the following items and 50,000 gold:

• Black Mushroom - located in a patch of mushrooms in level 1 of the Cathedral (Act 1)
• Shinbone - found occasionally in the fireplace of Leoric's Manor (Act 1)
• Wirt's Bell - can be purchased from a vendor in Act 2 for 100,000 gold
• Liquid Rainbow - can be found in a Mysterious Cave (random dungeon) in Dahlgur Oasis during Act 2
• Gibbering Gemstone - random drop from Chiltara in the Caverns of Frost (Act 3)
• Plans for the Staff of Herding - random drop from Izual (Act 4)

Once you have the staff, travel to New Tristram, and follow the road towards Old Tristram. You will find a fissure in the ground with a dead cow and it's spirit nearby. With the staff, you can open the fissure to enter the secret level.

It is Easter Egg referencing to a Diablo 1 and part of a "key" for the possible diablo 3 secret level.

"The Witch Adria seeks a Black Mushroom? I know as much about Black Mushrooms as I do about Red Herrings. Perhaps Pepin the Healer could tell you more, but this is something that cannot be found in any of my stories or books."

This is the complete line from deckard cain in diablo 1 Here is the complete quest dialogue.

Possible usage: since the secret cow level in D2 used a D1 easteregg in the crafting recipe (Wirt's leg), and the Black Mushroom is another D1 easteregg,It could lead to the new "secret level" of Diablo 3.

Now since full release of the game you can use the mushroom with other ingredients to create the "staff of herding" full guide here.The use for the staff is still unknown bit no doubt it will be something fun.