Oxygenated blood is bright red, whereas deoxygenated blood is a darker red. And it looks blue because blue light doesn’t penetrate the skin as deeply as red light. The ones closer to the surface appear blue while deeper ones are purpleish due to the red light reflecting deeper.
The actual acid (acetic in vinegar, citric in citrus and tomatoes) actually boils off with the water. So a long simmer actually removes the acid and changes the pH of the dish.