At home: Combine sugar, cornstarch, and salt in a small zip-top bag. Pack berries, crushed wafers, and marshmallows separately in bags or containers.
In camp: Place blueberries in a small pot with ¾ cup water, bring to a boil, and simmer until plump and soft, adding small amounts of water as needed. When the berries are fully reconstituted (about six minutes), there should still be some liquid in the pot. Remove from heat, add sugar mixture, and stir quickly to incorporate and minimize lumps. Cook over a medium-low flame, stirring constantly until mixture bubbles and thickens (about 90 seconds). Meanwhile, have your campmate toast the marshmallows until golden-brown. Portion berries into two bowls, cover each with three marshmallows, then top with wafer crumbs.