From the Super Mario Wiki
Not to be confused with B. Locker.
Blockers in Mario vs. Donkey Kong: Mini-Land Mayhem!

Blockers are objects that appear in Mario vs. Donkey Kong: Mini-Land Mayhem! They appear as tan rectangular dotted outlines, but turn into metal blocks that can be used as either floors or walls when tapped with the player's stylus. They function similarly to Red Girders, but they are not semi-solid even when placed horizontally. They first appear in a tutorial for the introduction of Cosmic Adventure, but are not seen in an actual level until Area 6-2. Blockers are also available for use in the Construction Zone game mode.

MLM Blocker Activated.png
MLM Blocker Deactivated.png