Lightly spray 6 (3/4-cup) ramekins or custard cups with nonstick cooking spray and set aside.
In a heavy, medium saucepan, combine the cream and milk. Scrape the seeds from the vanilla bean into the saucepan and add the scraped bean. Bring to a simmer over medium heat. Remove the saucepan from the heat and add the lemon zest. Cover and let milk mixture steep for 30 minutes. Strain mixture through a fine sieve and discard solids. Return the mixture to the saucepan.
Combine the lemon juice and gelatin in a small bowl and let stand until the gelatin softens, about 10 minutes. Add the gelatin mixture and the sugar to the cream mixture in the saucepan. Heat over low heat, stirring, until the gelatin and sugar are dissolved, about 2 minutes. Remove from the heat and whisk in the creme fraiche. Carefully divide the mixture evenly among the prepared ramekins. Allow to cool to room temperature then cover with plastic wrap and refrigerate until thoroughly set, at least 6 hours or overnight.
When you are ready to serve the panna cottas, run a small sharp parking knife around the edge of each custard. Dip the bottom of each ramekin into a bowl filled with hot water for 30 to 45 seconds to loosen the custards. Place a small dessert plate onto the top of the ramekin. While holding the plate and ramekin together, invert both, shaking gently to help release the custard from the ramekin. Serve the panna cotta with the blueberry sauce spooned over the top. Garnish with a dollop of creme fraiche if desired.
Fresh Blueberry Sauce:
Combine blueberries, lemon peel, sugar, and water in a medium saucepan. Bring berry mixture to a boil over medium-high heat. Reduce heat to medium-low and simmer for 5 minutes. Strain the mixture through a fine mesh sieve, pressing on solids to release as much liquid as possible. Discard solids and return sauce to a clean saucepan. In a small bowl, combine the cornstarch and water, stirring until smooth. Add the cornstarch mixture to the blueberry sauce, stirring until the sauce comes to a boil. Allow to boil for 1 minute. Remove from the heat and cool sauce to room temperature. Serve with Lemony Panna Cotta.