You have to edit the file:

/generators/arduino/blocklyduino.js

and modify the line 93:

profile[“default”] = profile[“arduino_uno”];

changing arduino_uno by arduino_mega.

You have to modify the file /utils/blocklyDuino.js at line 479.

You can find the list of blocks categories at LoadIds = line:

// set the default toolbox if none in session
if (loadIds === undefined || loadIds === "") {
loadIds = "CAT_LOGIC,CAT_LOOPS,CAT_VARIABLES,CAT_FUNCTIONS,CAT_MATH,CAT_TEXT,CAT_ARDUINO,CAT_TECHNOZONE";
window.sessionStorage.toolboxids = loadIds;
}

You can find the categories list name to write at line LoadIds in the file index.html starting line 246 in “toolboxes definitions” .