Empty Block

An Empty Block (sometimes referred to simply as a Block or a Steel Block ) is a type of block in the Mario franchise that usually appears when Mario, Luigi, and their friends hit a ? Block, an item-holding Brick Block, or a Coin Block, though they sometimes are naturally already in an "empty" state.

Super Mario Bros. / Super Mario Bros.: The Lost Levels
Empty Blocks debut in Super Mario Bros. and recur in Super Mario Bros.: The Lost Levels. In these games, they are colored to match the level's theme, like Brick Blocks and ? Blocks.

Super Mario Bros. 3
After an absence in Super Mario Bros. 2, Empty Blocks return in Super Mario Bros. 3; both these blocks and ? Blocks are yellow (brown in remakes) in this game. Big Empty Blocks also debut in this game, being the result of hitting Giant ? Blocks; they also form part of the level designs of Worlds 4-2 and 4-5.

Super Mario Land
Empty Blocks return in Super Mario Land, acting as in prior games. They also occur naturally empty, mostly as bonus room terrain.

Super Mario World
In Super Mario World, Empty Blocks have small, closed, angry eyes in their sides' centers, rather than their usual holes. If the player hits a blue Switch Block, all Empty Blocks turn into coins and vice versa for a short time.

Super Mario Land 2: Six Golden Coins
In Super Mario Land 2: Six Golden Coins, Empty Blocks, looking like they do in Super Mario World, appear in the same capacity as in prior games.

New Super Mario Bros. series
Empty Blocks return in the New Super Mario Bros. games, working as they did previously; they are color-coded according to their environment, as in Super Mario Bros. They can now be broken by Mega Mario (in New Super Mario Bros. and New Super Mario Bros. 2) or stronger enemies, such as Broozers (in all games), King Bills (in New Super Mario Bros. Wii and New Super Mario Bros. U), and Super Bowser (in New Super Mario Bros. Wii).

Big Empty Blocks also return in red and orange Toad Houses in New Super Mario Bros. (as a result of hitting their respective blocks), World 6-5 in New Super Mario Bros. Wii, and Soda Jungle in New Super Mario Bros. U; Long Empty Blocks return in New Super Mario Bros. 2.

Super Mario Galaxy / Super Mario Galaxy 2
Empty Blocks appear in a 3D Super Mario game for the first time in Super Mario Galaxy, and they return in Super Mario Galaxy 2; however, they are always found naturally empty, as Brick Blocks always shatter when hit, even if they contain items, and ? and Coin Blocks vanish once used up.

Giant Empty Blocks also appear in the Supermassive Galaxy in Super Mario Galaxy 2, where they are part of the stage's level design.

Super Mario 3D Land
Empty Blocks return in Super Mario 3D Land, working as they do in the 2D games. Giant Empty Blocks are also found in hidden rooms accessed via Warp Pipes (as well as in World 6-3) and are often put together to form sculptures that create 3D illusions.

Long Empty Blocks also debut in this game, as a result of hitting Long ? Blocks.

Super Mario 3D World
Normal-sized and long Empty Blocks return in Super Mario 3D World, acting as they do in prior games. Certain ? Blocks also produce lines of Empty Blocks when hit, similar to face blocks from Super Mario 3D Land; giant versions of these blocks (though with a normal-sized "?" mark) also appear and make Giant Empty Blocks.

Super Mario Maker / Super Mario Maker for Nintendo 3DS / Super Mario Maker 2
Empty Blocks return in Super Mario Maker, Super Mario Maker for Nintendo 3DS, and Super Mario Maker 2, working as they do in the New Super Mario Bros. games. In Super Mario Maker 2, giant Empty Blocks also appear in the Super Mario 3D World style, as ! Blocks take the role that Mega ? Blocks did in the actual game.

Super Mario Odyssey
Only normal Empty Blocks reappear in Super Mario Odyssey. They can be found naturally empty in 8-bit sections.

Paper Mario series
Empty Blocks appear in all Paper Mario games, working as in the main series. Giant Empty Blocks return in Mondo Woods in Paper Mario: Color Splash.

Super Smash Bros. series
Empty Blocks also appear on Mario-themed stages in the Super Smash Bros. series.

Super Smash Bros. Melee
They first appear in Princess Peach's Castle in Super Smash Bros. Melee, if the red, blue, or green ! Blocks are touched or hit (or if items are turned off).

Super Smash Bros. Brawl
In Super Smash Bros. Brawl, ? Blocks on Mushroomy Kingdom become these if hit or jumped into from below, and they are refilled when the stage loops; they are made of wood with metal borders, and like the rest of the stage, they look more realistic.

Super Smash Bros. for Nintendo 3DS / Wii U
In 3D Land in Super Smash Bros. for Nintendo 3DS and Super Mario Maker in both the 3DS and Wii U versions, they act the same as in Brawl but have their respective appearances; Empty Blocks are instead refilled after a while on the latter stage.

Super Smash Bros. Ultimate
Super Smash Bros. Ultimate brings back not only the aforementioned stages, but also the Mushroom Kingdom stage from Super Smash Bros.; in this version of the stage, the pass-through platforms are made of Empty Blocks, rather than Hard Blocks like in the original version. Empty Blocks are also embedded into the brick wall at the left side of the stage, and they are also used as the paths and ledges fighters walk and grab on.

The hazardless versions of the stages also have all ? Blocks naturally empty.

Mario & Luigi series
Empty Blocks appear in the Mario & Luigi series, acting the same as in the 2D games.

Mario Kart series
Empty Blocks also appear in the Mario Kart series, usually as part of a track's wall or boundaries.

Mario Kart 7
In Mario Kart 7, they appear as parts of platforms that can be driven on in Piranha Plant Slide.

Mario Party 9
In the Mario Party 9 minigame Fungi Frenzy, ? Panels turn into panels resembling Empty Blocks after they are ground-pounded.

Trivia

 * In the Super Mario Mash-Up Pack in Minecraft, Bedrock is replaced by Empty Blocks.