Lemon juice contains citric acid and is acidic to taste, however, lemons are considered alkaline forming. This means that the effect on the blood is alkaline versus milk that don't taste acidic, yet has acid forming properties in the blood.
Acid is anything below a pH of 7, 7 is neutral, and above 7 is alkaline. Our blood must stay between a pH of 7.35 and 7.45 which is slightly alkaline. There is a greater tendency to head towards acidosis i.e. too much acid in the blood, due to diet and lifestyle factors that are more prone to acid formation. Minerals such as calcium and magnesium that are required for strong bones, are alkaline minerals and are frequently needed to bring our blood back to neutral. Hence diets that are too acid forming (especially high protein and fried foods), can lead to conditions such as osteoporosis.