I stole this from a friend of mine who also grows..... Calyx swelling is a major indicator of peak maturity. The lowest, oldest calyxes swell first and the swelling works its way up to the highest, youngest flowers on each bud. At peak maturity about 90% of the calyxes will almost look seeded, they are so fat. Honestly, I'm not entirely sold on any swollen calyx theory. I currently have about a half dozen or so on my plants, but as long as they keep kickin' out nice looking buds, I'm totally cool with it. Good luck!
Oh and kudos for getting your hands on blue dream. It is absolutely my favorite strain, but I can't find it in seeds or clones anywhere around here.
I've had great results with their seeds. Got a couple going now. My GSC is just now flowering. Looking beautiful.