Bring the orange and lime juice as well as the lime zest, agave syrup and two sprigs of lemon thyme to the boil in a small saucepan. Set aside to cool.
Peel, core and dice the melons. Rinse and drain blueberries.
Remove the thyme sprigs from the dressing, pour it over the fruit and let it steep in the refrigerator for about an hour, covered.