Blackberries are not considered to be citrus fruits because their total acidity level is not high enough.
Some of the fruits that are considered to be citrus fruits include oranges, grapefruits, limes, and lemons. All of these fruits have a high enough acidity level on the pH scale to constitute this classification.
Those who suffer from heartburn/acid reflux will be able to safely eat these berries, as there is no chance of aggravating this condition.
In fact, blackberries are considered to be a very low alkaline fruit because they contain such little acid overall and are ranked extremely low on the pH scale which is used to measure acidity in foods.