If a person experiences stomach pains, vomiting, diarrhea, or mild rashes after ingesting even modest quantities of blueberries, they may be allergic to the fruit. Typically, stomach pains are caused by the inability of the intestines to digest the fruit. Vomiting or diarrhea may be an indication of the body's rejection of blueberries as a valid nutritional source; essentially it has identified blueberries as a potential toxin and it is trying to limit exposure to blueberries. Rashes are typical when applying a topical ointment with blueberries and/or blueberry extracts but can also occur when ingesting blueberries in some cases. Rashes are the indication of elevated histamine responses; histamines are your body's response to allergens and is an over-reaction on the part of your immune system. Anti-histamines can alleviate the immediate symptoms.