Black plums are not acidic, in the sense that they don’t have a pH that’s well below 7. They’re a little bit acidic though- a lot of fruits with higher levels of sugar can be acidic, because acids cause tooth decay and metabolism slows when someone is afflicted with them.
Black plums contain about 4% acidity. Apples, which many people think to be more “acidic,” actually have an acidity content of 11%, though blackberries go up to 12%…