As far as I know the body maintains blood ph between 7,35 and 7,55. If it is out of this range the body dies. Body uses calcium to maintain that correct level.
If you diet is too acid (Meat and meat products - Uric acid; Pork - sulfuric acid and nitric; Sweets, sugar, white flour products - acetic acid; Sweetener - formic acid; Soft drinks (carbonated and sweetened) - phosphoric acid; Coffee, tea, red wine - tannic acid; Painkillers - acetylsalicylic acid; Excessive physical exertion - Lactic acid, or you stress or angry a lot (hydrochloric acid) your body will be overacid. 99% of body calcium is stored in bones so if you are overacid body will use that calcium (it leads to osteoporosis) and store calsereous salts in joints (it leads to joint pain and artritis). The other important mineral is magnesium because it helps calcium to become ready to use. Potassium also plays an important role in balancing body pH.
It makes sense to me that just eating more alkaline food would not resolve joint pain. What should help is improving level of minerals, especially magnesium deficit which is impossible to be done from food. The resulting calcium - magnesium ratio should be 2:1.