Clinical research has found that polyphenol rich foods such as Blueberries may improve mood and some measures of cognitive performance.
Blueberries are an excellent source of polyphenolic compounds such as anthocynanins, which have potent antioxidant and anti-inflammatory properties.
Blueberries are also a good dietary source of Vitamin C, Vitamin K, manganese, dietary fiber and a large number of phyto-chemicals.
Consumption of blueberries and/or anthocynanin rich foods are associated with a reduced risk of cardiovascular disease, death, and type 2 diabetes, and with improved weight maintenance and neuroprotection.
Epidemiological studies indicate that the consumption of polyphenol rich fruits and vegetables is associated with a lower risk of developing neurodegenerative diseases and better cognitive function in the elderly.
Wild blueberry diet supplementation was proved to improve cognitive function in older adults.
Increased consumption of berries and anthocyanins, as well as total flavonoids, was shown to be associated to a slower progression of cognitive decline in a large prospective cohort of older women. 
Blueberry & Cognitive Peformance
Numerous studies have found that various blueberry interventions can improve cognitive function.
A systematic review of randomized controlled trials published 2020 investigated the effect of blueberry interventions on cognitive performance and mood.
The systematic review concluded:
Based on the current evidence, blueberries may improve some measures of cognitive performance. 
Another systematic review published 2019 evaluated the existing clinical research from both acute and chronic blueberry interventions on cognition in human subjects.
The results of 11 studies are reported with 4 studies considering blueberry intervention with children aged 7-10 years, 4 considering adults aged 60 years and older, and 3 considering adults suffering from mild cognitive impairment (MCI).
Findings from these studies indicate that cognitive benefits may be found for delayed memory and executive function in children and for delayed memory, executive function, and psychomotor function in older healthy and MCI adults. 
The information in this article has not been evaluated by the FDA and should not be used to diagnose, cure or treat any disease, implied or otherwise.