Hidden Block

Hidden Blocks (also called Invisible Blocks ) are ? Blocks that are invisible to players unless they hit the blocks, which turns them into Empty Blocks.

Super Mario Bros. / Super Mario Bros.: The Lost Levels
The first Hidden Block, which contained a 1-Up Mushroom, appears after the fourth pipe in World 1-1 in Super Mario Bros.. Similarly, Hidden Blocks with 1-Up Mushrooms in them can be found in each world's first level, so long as the player collected all of the Coins in the previous world's third level. Otherwise, most Hidden Blocks only hold a single Coin. Also, Super Mario Bros. Deluxe's Challenge Mode adds a Hidden Block holding a Yoshi Egg to each level.

In Super Mario Bros.: The Lost Levels, Hidden Blocks are more often placed to make avoiding enemies more difficult. They are also used to provide viable paths or platforms through courses, usually castles. This is also the first game where Hidden Blocks can contain power-ups, as well as the newly introduced Poison Mushroom.

Super Mario Bros. 3
After an absence in Super Mario Bros. 2, Hidden Blocks return in Super Mario Bros. 3. They are often used to lead players to secret areas and items. In cave levels, Hidden Blocks' locations are revealed by the different dot coloration (only in the NES version).

Super Mario World
In Super Mario World, Hidden Blocks return. However, certain Hidden Blocks will only be visible and usable when a P Switch is activated.

New Super Mario Bros. series
In the New Super Mario Bros. series, Hidden Blocks return and work as they used to. Some of them may also hold Beanstalks. In particular, a Hidden Block containing a power-up can be found near most of the big Warp Doors leading to the tower and castle bosses.

Super Mario 3D Land
Hidden Blocks make their debut in a 3D Mario platformer in Super Mario 3D Land. In this game, some Hidden Blocks can slightly be seen if the player has the 3D function on.

Super Mario 3D World
In Super Mario 3D World, the locations of Hidden Blocks can be briefly revealed by ground pounding or by touching the screen or blowing into the microphone of the Wii U GamePad. Wide Hidden Blocks also debut in this game, acting like wide ? Blocks.

Super Mario Maker / Super Mario Maker for Nintendo 3DS
In Super Mario Maker and Super Mario Maker for Nintendo 3DS, Hidden Blocks were given a sprite for use in Create mode, as a translucent gray ? Block.

Super Mario Odyssey
Hidden Blocks appear in Super Mario Odyssey, working as they did in Super Mario 3D World. When Mario puts on a Moe-Eye's sunglasses while he is controlling it, he is able to see Hidden Blocks.

Super Mario RPG: Legend of the Seven Stars
In Super Mario RPG: Legend of the Seven Stars, Surprise Boxes are invisible Treasure Boxes that contain various items from Frog Coins, Flowers, powerful healing items, and items used in battle. When Mario jumps at a spot where a Surprise Box is, it first reveals itself, and jumping at the revealed Surprise Box allows Mario to collect its contents. There are 39 Surprise Boxes in the game.

Paper Mario
In Paper Mario, the player may come across Hidden Blocks containing coins, healing items, badges, and other items, while some are also used as stepping stones to reach other areas. Such blocks can be found by using Watt's field ability. There are 47 Hidden Blocks in the game.

Paper Mario: The Thousand-Year Door
Hidden Blocks function the same way in Paper Mario: The Thousand-Year Door. The player can use Ms. Mowz's field ability to find them. There are 15 Hidden Blocks in the game.

Mario & Luigi: Superstar Saga / Mario & Luigi: Superstar Saga + Bowser's Minions
Hidden Blocks appear in Mario & Luigi: Superstar Saga, containing the elusive Hoo Beans. Such blocks are found by jumping around at suspicious platforms or patterns, or in seemingly "empty" dead ends. There are 42 Hidden Blocks in the game; a list of them can be found here. In Mario & Luigi: Superstar Saga + Bowser's Minions, the hidden blocks are necessary for 100% completion, as they are added to the block total. They also now gain a distinctive shine over them every once in a while, making their exact location a bit easier to find. There is also an item called the Secret Specs in the remake that makes them even more visible, and can be obtained at the Starbeans Cafe after brewing a fifth unique blend of Bean Juice.

Mario & Luigi: Bowser's Inside Story
Hidden Blocks appear in Mario & Luigi: Bowser's Inside Story in Peach's Castle Garden, containing the last three Attack Pieces of the Special Attack, Falling Star. They also appear in Toad Town Mall during the Mushroom Ball Derby sidequest, in which case they contain Mushroom Balls. In both cases they turn into empty blocks, but those in Toad Town Mall disappear soon after the player obtains the Mushroom Ball. Some also appear in the Flab Zone to allow Mario and Luigi to access certain otherwise unreachable platforms.