Place tomatoes, carrots, and onion in a stockpot. Bring to a boil over medium-high heat; reduce heat to medium-low and simmer for 30 minutes. Stir in chicken broth, sugar, and salt.

Melt butter over medium-low heat in a small saucepan. Whisk in flour to form a thick paste; gradually add milk, whisking continuously until thickened and smooth, about 5 minutes.

Stir milk mixture into the stockpot. Season with basil, celery salt, black pepper, and garlic powder. Continue to simmer the soup on low to reduce and thicken, about 1 hour.