Beta-hydroxtbutyrate is fuel for the fat adapted brain. Eat foods that put you in nutritional ketosis. Carnivore is not automatically ketotic due to variations in protein and insulin effect. So, it’s worth checking blood ketones.
People eating Low Carb Healthy Fats that at various times in the day put the body into nutritional ketosis, especially combined with limited feeding windows, can probably get away with eating walnuts and blueberries in measured amounts (calculating carbohydrate dosage). It doesn’t make them brain foods. There may be some micronutrients in them that have been shown by reductionist biochemists to have cellular effects if we ignore the complexities of human biology and human physiology.
There will even be people preaching that the brain needs glucose and base their brain food ideas on that misconception (we know that blood glucose that becomes brain glucose need not be dietary carbohydrates).
Then to go for a therapeutic effect, i.e. resolve brain fog, treat it like a treatment and aim for a blood glucose:ketone index of 1:1 (GKI) trialling something like the Paleolithic Ketogenic Diet (PKD) and compare it against what you are doing now.
A thing that looks like brains (like walnut kernels do) that might help your nutritional ketosis experiments is actually brains. People say lambs brains are nice to eat.