Bring blueberries, sugar, and salt to a simmer in a 4-qt. saucepan over medium-high heat. Reduce heat to medium and cover; cook, stirring occasionally, until most of the berries burst and release their liquid. Stir in lemon juice, butter, and thyme; continue to cook, uncovered and stirring continuously, until jam is thick, about 2 hours. Discard thyme sprigs. Follow canning procedures or let cool and refrigerate for up to two weeks.