Concerning blood pH, I would like to point out that the body keeps its blood pH very constant, so it never varies much. If your blood pH drops below 7.3 or rises above 7.4 your body has a very effective mechanism of keeping it in check with a system that involves buffers, kidneys and lungs. For example if the blood ph starts to become acidic (under 7), immediately you will start breathing a bit faster to get rid of CO2, and thats makes your blood pH go up very quickly.
So I do not think that blood pH is a factor regarding psoriasis, I wouldn't waste my time thinking about that and testing yourself etc.