There can be various reasons for this but in your case, I'd suspect a nutrient deficiency based on the pH. If the soil pH is too high - greater than 5.5 - then the uptake of phosphorus is impacted. The result is that purple or red coloration. Excessive nitrogen or nitrate based N can also have a similar effect.

Since the pH of your tap water is so high, I'd also suggest acidifying it with every watering. A couple of tablespoons of plain old table vinegar per gallon of water will keep the pH down to blueberry acceptable levels :-)

I agree with gardengal, but I would use only 1 tablespoon and check the water using one of those pH strips. They seem to be more accurate than cheap meters. With my well water one tablespoon puts the pH in the 5.5 range according to the strips.