Hi Daisydot,
Just a quick reply as I'm at work.
In a nutshell, the pH of your blood only fluctuates very slightly. The kidneys perform an important role in maintaining the correct pH balance of your blood, and they do that by excreting excess acid in the urine. That's why your urine can be acidic even though your blood isn't.
Regarding gout- a diet low in purines is recommended because the body converts purines to uric acid. So if someone with gout eats fewer purine-containing foods, they make less uric acid and so have less of a problem with uric acid crystals in their joints.
It's not to do with the acidity of the purines.
Hope that helps, but I can explain further tonight xx