Anytime I see a field with a dogbane problem, I say the best control is to plant the field to alfalfa.  4-5 years in alfalfa, mowing 4-5 times per year, and that dogbane is gone.  I believe the real key there is frequent mowing.

Short of planting alfalfa, I think the key to dogbane control with chemicals is timing.... and persistance.  As noted above, spray in the fall, when the plants are sending energy down into the roots for winter.  The other best time to spray them is when they are in bud-to-bloom stage.  For us, that occurs sometime around June.  If you try to spray them before that time, I believe you are just wasting your time. 

Again, one shot is not going to kill them all.  It takes time, and multiple applications to get this weed under control.

Paraquat is not going to do anything for you, except make you feel like you did something that day.

Crossbow would be effective; or adding some Banvel/ Clarity to the glyphosate would really help.  However, I would be leary of using Crossbow or Banvel/Clarity around blueberries.  Right now, I think the best you can do is glyphosate - and watch your timing.