Blood pH and diet

Blood pH is relatively constant at around 7.4 because the body will strip calcium from bones, if necessary, to buffer it in the absence of alkaline-forming food components. That is not homeostasis. But blood pH does vary (typically within a range of 6.8 to 7.8, with potentially lethal effects beyond that range) and affects urine pH, which we can easily measure. 

Dietary compounds formed in the body affect blood pH; not the contrasting pH levels in the digestive tract (acid in the stomach and neutral in the small intestine) or the apparent acidity of foods, but what happens after digestion. The minerals or weak acids (citrates, et al) in alkaline-forming foods are used to make alkaline compounds in the body, which won't happen efficiently with a processed food or a non-plant-derived diet. Scientists have measured this effect and doctors have utilized it in practice, so it's not theoretical or based on pH limited to areas of the digestive tract. https://www.drrusselljaffe.com/7-principles-eating-alkaline-way/ 

"An advanced understanding of acid–base physiology is as central to the practice of critical care medicine, as are an understanding of cardiac and pulmonary physiology. Intensivists spend much of their time managing problems related to fluids, electrolytes, and blood pH. Recent advances in the understanding of acid–base physiology have occurred as the result of the application of basic physical-chemical principles of aqueous solutions to blood plasma. This analysis has revealed three independent variables that regulate pH in blood plasma. These variables are carbon dioxide, relative electrolyte concentrations, and total weak acid concentrations. All changes in blood pH, in health and in disease, occur through changes in these three variables. Clinical implications for these findings are also discussed." https://www.ncbi.nlm.nih.gov/pmc/articles/PMC137247/