Surfactant is product that does the same as diesel only MUCH more environmentally friendly. Basically, its job is to help herbicide stick to whatever you are spraying.
For blackberries you really don't need any, as the leaves are not waxy and liquids stay on them pretty well. Never hurts though and about 2 tablespoons per gallon is fine.
As to the mix, the container should give you the water to herbicide ratio.
Be real careful of drift with that stuff.
Good luck!
I use 2-4D (Weed-Be-gone) on lawn weeds. I prefer to spot spray if I can but I also spay the entire lawn sometimes. I generally use a little dish washing liquid as a surfactant. If that's no okay, why not? As infrequently as I use the stuff I've never noticed any problem.