I agree that saliva ph is not a good indicator of blood ph. From what I know, the blood ph can't vary too much from 7.4, otherwise you die immediately.
The believer's in saliva ph say that it's an indicator of the ph of the extracellular (or non-blood related) fluids in the body. When i measure my ph first thing in the mornings, it consistent tends to be around 5.5 or less, regardless of what I ate, and drank the night before, or whether I brushed my teeth or not. So, it feels that perhaps there is more to the saliva ph.