OK, it looks like the reason is really just 2 different strains. I don't grow in soil and I haven't done any sativa dominant plants, however the many I have seen all are stretchy like yours and the colas are kind of separated on the branches. Your plants look healthy and the purple is just a slower developer. Look back and compare some of the pics from the past 10 days and compare how the blueberry looked right before it started to make a lot of pistils.
i know on the plants I have grown and I was told by folks on here that when the leaves stop coming in pairs and come every other leaf that is a sure sign of flowering.
I use litmus paper all the time, it's handy for a quick check. There is a color chart for litmus paper for pH. Might try Google.
Happy farming! 🙂🌱