Blue Coin

Blue Coins are a special type of coin that first appeared in Super Mario Bros. 3, and have since appeared in many later games. They typically appear in large groups after a switch is pressed, and may vary from a light cyan to a deep, heavily-saturated blue.

Super Mario Bros. 3
Blue Coins first appear in Super Mario Bros. 3, where they appear in certain spots for as long as a P Switch is in effect. Blue Coins have the same value as standard Gold Coins and are found usually in clusters. They have a light cyan color in all versions, but are mistakenly described as "silver coins" in strategy guides.

Super Mario World
Blue Coins were originally going to appear in Super Mario World, where they would turn into magenta blocks upon a switch being hit.

Super Mario 64 / Super Mario 64 DS
Blue Coins later appear in Super Mario 64 and also made an appearance in Super Mario 64 DS. In these games, their color is darkened from the light cyan ones in Super Mario Bros. 3.

In these games, Blue Coins are worth five Yellow Coins each. They can usually be found by Ground Pounding a Blue Switches, which, similar to the P-Switches, will cause a number of Blue Coins to appear for a limited amount of time. Blue Coins can also be collected by defeating certain enemies, such as Koopa Troopas, Piranha Plants, Boos, Mr. Is, or Pokeys. Big Goombas also give out a Blue Coin if killed using a Ground Pound. Bookends will provide a Blue Coin when they are attacked, but not if they disappear after colliding with the floor, a wall, or the player.

Blue Coins are also found in the slides on Cool, Cool Mountain (one is present) and Tall, Tall Mountain (three are present), where they will start to move away from Mario once he gets close to them, in contrast to the stationary Yellow Coins. However, the Blue Coins in Tall, Tall Mountain are not present in Super Mario 64 DS.

Super Mario Sunshine
In Super Mario Sunshine, Blue Coins appear as a rare, collectable type of coin. There are 240 Blue Coins total in the game (30 in each of the seven courses, a combined 20 in Delfino Plaza and Delfino Airstrip, and 10 in Corona Mountain). By heading to the Boathouse located in the west part of Delfino Plaza, the player can trade ten Blue Coins for one Shine Sprite from the Raccoons running the store, meaning 24 Shine Sprites total are available using this method. They do not affect the number of Yellow Coins that Mario has collected; however, they restore two health points when collected. The player receives the option to save their game for each Blue Coin they grab.

There are some Blue Coins (as well as Yellow Coins) in Sirena Beach's hotel that are fake since they do not spin and move. They are actually Boos with coins inside of them, and if Mario gets too close to a "coin", the Boo appears.

New Super Mario Bros. series
Blue Coins appear in New Super Mario Bros., having the same role and cyan color as in Super Mario Bros. 3. They sometimes appear in a trail behind Invincible Mario in a few levels, like World 7-3 and in a secret area in World 5-3.

Blue Coins reappear in New Super Mario Bros. Wii, now with a slightly-deeper color than in New Super Mario Bros. When the player collects them all in a level before they disappear, the audience cheers. They are often arranged in patterns.

Blue Coins also appear in New Super Mario Bros. 2 (but when the player collects them all, the audience instead claps) and New Super Mario Bros. U. In the latter game's expansion pack New Super Luigi U, a Blue Ring which produces many Blue Coins similar to P-Switches also appears.

Super Mario 3D World
Blue Coins reappear in Super Mario 3D World, where they serve the same function as in the New Super Mario Bros. series.

Mario Pinball Land
In Mario Pinball Land, these coins can be obtained by making a combo without using the flipper. Mario can use them to play minigames from Toad for Power Stars.

Mario & Sonic at the Olympic Winter Games (Wii)
In Mario & Sonic at the Olympic Winter Games, Blue Coins appear after the player buys the blue treasure chest in the Secret Shop. Getting all five Blue Coins will result in the player getting an emblem. The events that blue coins appear in are Dream Alpine, Dream Ski Jumping, Dream Ski Cross, Dream Snowboard Cross, Dream Short Track, and Dream Bobsleigh.

Super Smash Bros. for Nintendo 3DS
Blue Coins appear in the Golden Plains stage exclusive to Super Smash Bros. for Nintendo 3DS, after a fighter steps on a P-Switch that occasionally appears during a match. They serve the same function as the normal Coins in the stage.

Mario + Rabbids Kingdom Battle
In Mario + Rabbids Kingdom Battle, they appear in the bonus stages. Collecting all of the Blue Coins will reward the player with a new weapon.

Trivia

 * The functionality of Blue Coins in Super Mario 64 was likely carried over from the development of Super Mario World, as in that game Red Coins, which went unused, also awarded the player with five Yellow Coins when collected.