Butter a 9×13 baking dish. Cut 1 1/2 inch slices from the baguette and arrange in one layer in the dish (about 6-8 slices).
Whisk together eggs, milk, nutmeg, vanilla, and 1/2 cup brown sugar in a large bowl and pour evenly over the bread. Refrigerate mixture, covered, until all liquid is absorbed by the bread, at least 8 hours and up to 1 day.
When you are ready to bake and serve, preheat oven to 350 F. Spread pecans evenly in a shallow baking pan and toast in middle of oven until fragrant, about 4 minutes. Toss pecans with 1 teaspoon butter. Alternately (and this is what I did), toast the pecans in a skillet in one layer over medium heat, stirring often. Toss with 1 teaspoon butter. (I did this the night before to save time the morning of).
Sprinkle pecans and blueberries evenly over bread mixture. Cut remaining 1/2 stick butter into pieces and heat with remaining 1/4 cup brown sugar in a small saucepan, stirring, until butter is melted and sugar is dissolved.
Drizzle butter mixture over bread and bake mixture 30 – 40 minutes or until any liquid from blueberries is bubbling.
Notes
Plan Ahead: be aware that the bread needs to soak in the egg/milk/brown sugar mixture up to 8 hours so plan ahead!