Preheat oven to 450 degrees F.
Put the tomato and lemon juice in a food processor or blender and puree it. Transfer the tomato puree to a medium saucepan and heat over medium heat, bringing it to a low boil. Add the celery, lemon zest, sugar, cinnamon, nutmeg, and water. Reduce the heat to low and simmer until the sauce is thick, like a marinara sauce. Remove from the heat and gently mix in the blackberries. Add salt, to taste.
Lightly flour both sides of the trout and sprinkle it with salt and pepper. Heat a large skillet over medium-high heat and add enough olive oil to sear the trout. When the oil is hot, add the trout and sear on both sides just until brown. Place the skillet in the oven and continue cooking until the fish is flaky, about 10 minutes. Top with the sauce and serve immediately.