I've made this twice now. Turned out silly good both times. I would add most of the blueberries in primary. Save maybe 1 lb and add them in 2ndary. More than that, and theres so many in the carboy, they just sit on top and don't steep in the wine.
Oh, and I froze my blueberries, thawed them, and smashed them in a plastic bag before dumping them in a mesh fruit bag for primary. For 2ndary, just dump a pound right into the carboy and rack the mead over them. Leave a little space as the berries tend to expand and blow up your airlock.
I usually start with a 5.5-6 gallon batch, so after primary, I have an extra half-gallon jug I use to top up the carboy after racking out of 2ndary. I hate topping up with water. Just my two gold pieces, from a seasoned winemaker of 18 months :-D