M Block

From the Super Mario Wiki

A red M Block found in the Mario & Luigi sub-series serves the same function as a ? Block. This type of block can only be activated by Mario and Baby Mario, whereas the similar L Block can only be used by Luigi and Baby Luigi.

M Blocks also appear in Yoshi's Island DS. In this game, Yoshi must be carrying Baby Mario to see them and hit them. Also, some remain invisible even with Baby Mario; these blocks must be hit by Yoshi (with Baby Mario) to appear. The stage Hit the M Blocks! is named after them.

Blocks

! Block | ? Block | ♪ Block | 4-Bros. Random Block | 99 Block | Arrow Block | Attack Piece Block | Aurora Block | Black Block | Blue Coin Block | Bonus Block | Boost Block | Brick Block | Bros. Block | Coin Block | Command Block | Control Block | Dice Block | Dotted Line Block | Drop Block | Egg Block | Event Block | Expansion Block | Face Block | Giant ? Block | Giant ! Block | Grab Block | Green Block | Grey Brick Block | Hidden Block | Hint Block | Ice Block | L Block | Laser Block | M Block | Marshmallow Block | Metal Block | Minus Block | Mushroom Block | Pink Block | Platform Block | Plus Block | POW Block | Pushing Block | Rally Block | Random Block | Recovery Block | Red Block | Rock Block | Save Block | Shine Block | Simu-Block | Slow Block | Snake Block | Speed Block | Spin Block | Star Block | Stone Block | Super Block | Triangular Block | Turning Block | Used Block | Vanishing Block | Warp Block | Watermelon Block | Wooden Block | Yellow Block | Yoshi Block