Halve vanilla bean lengthwise. Scrape the seeds into a medium saucepan. Add the pod, 2 cups blueberries, sugar, lemon zest, lemon juice and salt. Bring to a boil. Reduce heat to a low simmer and cook for 10 minutes.
Add the remaining 2 cups blueberries and cook over low heat for 5 minutes more. Serve warm or room temperature.
To make ahead: Refrigerate for up to 1 week; freeze for up to 3 months.