Q. How long?
A. Probably for a total of 8-9 weeks. Blueberry is all indica (maybe a little sativa, depending on the specific strain) and will finish flowering closer to the 8 week mark I'd imagine. Make sure you have something with strong magnification (good camera, 25x magnifying loupe, etc.) and check your trichomes; you will want them to be mostly milky white and not completely clouded. This should also be when most (70-90%) of the pistils have turned dark and are a nice deep amber.

Q. How long should I flush?
A. About a week. Some people do it longer, some shorter, but a week.

Q. Will the 6 plants still growing be a problem?
A. YES! If you have them in the same reservoir, Your remaining, growing sativas will be deprived of nutrients for the week that you flush. Find a way to separate them for the flush week, or don't and lose a week of growth. You should try to separate your plants based on nutrient cycles so that you do not run into this problem again.