From the Super Mario Wiki
Dark Trashures are dark purple forms of the Trashure. Found in Peach's Castle during the events of Mario & Luigi: Bowser's Inside Story, these enemies are stronger than their original forms, throwing faster and more harmful items and less Mushrooms and coins. Although they have more power, they are also more prone to running away and leaving the player with no EXP. Like normal Trashures, if the player does not counter the hammer from the Dark Trashure, the Mario Bros/Bowser may get stunned making them wait a few turns. They also have the highest health out of all non-boss enemies. They are mostly seen in battle with Dark Mechawfuls and appear to be working for the evil Fawful though this is unknown if they do.
Names in Other Languages
||From "oscuro" (dark) "cofre" (chest) and "fraude" (fraud)
||From "coffre-fort" (safe, "coffre" also means chest), the familiar word "fortiche" (clever) and "noir" (black).