Want to avoid berry desserts. I did a potato salad last month or I'd do a potato salad with the 3 colors of potatoes.
Make a trio of desserts (rice pudding, custard, ice cream) using cherries, vanilla and one of blue plums, blue grapes (especially concord) or blue Curacao.
Or tequila from the blue agave plant.
If no desserts, make savory noodle or bread puddings using tomato, cauliflower and blue carrot or blue cabbage.
Red white and blue corn corn chips.
A fruit salad bananas, raspberries and blueberries.
Or a platter of roasted eggplant slices, sliced fresh mozzarella and sliced tomatoes serve it with slices of a good, crusty bread.
How about steak (red meat), blue cheese and a creamy dressing.
Since blue food is hard to come by and not very appealing maybe use royal blue platters and then you can put white & red food on them. Pasta & tomato salad or mozzarella tomato salad on blue platters for example Even the blue potatoes are really deep purple in color. Desserts & drinks are easier thanks to food coloring. Good Luck!