Hi, I bought into the whole 'you have to alkalize your body' hype a few years back, but I have learned a bit since that time. First of all, I DO believe that once a body is in too much of an acidic state, it does create a state that is ripe for disease to develop but I don't believe in the hype of the 'alkalizing' diets and Alkaline Water and such things. You have already figured out the answer to your initial question, no the acidity of the urine does not have any correlation to the acidity of the blood. The blood will always be alkaline or you would be dead. Certain parts of the body will be acidic in a healthy state including the stomach and small intestine (probably the majority of the intestine is at least somewhat on the acidic side even in a healthy state).
That being said, acidosis of the body is an extremely harmful state for the body to be in. This basically means that throughout the entire body, the balance has been tilted toward being acidic. For me, the thing that has helped me understand all of this is Ayurveda (traditional Indian medicine). Understanding Ayurveda correctly is a quite difficult process as there is a lot of misunderstanding on the subject.
Basically, what I have learned is that the proper way to reverse the process of acidosis is not to follow some fad 'alkalizing' diet, but to work on the underlying issues and re-balance the entire body so that it functions optimally. The stomach and other digestive organs, especially the pancreas (which naturally produces sodium bicarbonate) when functioning optimally, should be able to produce proper alkalizing minerals.
As far as diet, meat is certainly acidifying, as is a high-fat diet), but I strongly disagree with the idea that grains and milk are acidifying to the body. Maybe in the short term (meaning within the first 6 hours of digestion or so) they could have an acidifying effect as the waste products of anything with and adequate amount of macro-nutrients will be at least somewhat acidifying to the body. But long term, the nutrients contained in wholesome whole grains and milk will actually help the body build its reserves of alkalizing minerals. Really, what the idea of the 'alkalizing' diet amounts to is to starve yourself, you eat tons of alkalizing vegetables that have almost no macro-nutrients, so sure you add tons of alkalizing minerals with basically no calories (so very little acidifying waste), you will alkalize your body initially, but totally destroy the balance of your body and probably deplete your body of a lot of minerals and nutrients that actually keep you healthy. Some of the 'alkalizing' fanatics will even tell you that most fruits are acidifying which is utterly ridiculous. At any rate, a balanced diet low in meat and high in grains, legumes, fruits and veggies is generally alkalizing, but if you really want to alkalize the body, that has to do with correcting the underlying imbalances and that requires a lot more research on your part.
As far as taking large doses of vitamin C goes, I am not a fan of this or taking anything that is separated from actual food. I don't believe natural vitamin C would ever be acidifying but supplementing with large doses of artificial (or at least separated from real food) is very likely to be acidifying.