I'm doing a 4 gallon batch of cider right now. Only difference is .75 lb of honey and a 1.0 lb of brown sugar. I'm using White Labs English Cider Yeast, Potassium Sorbate to "kill " the fermentation and racking to a secondary adding 2 cans of apple juice concentrate to keep a hint of sweet apple flavor. When it's done, I'll post the recipe. (If its good and not Gross!) I did a 5 gallon batch using 3 lb of each using champagne yeast and It came out hot (14% ABV) and really dry. I've still got 3 bottles left and its a year old. Good effervesense from the yeast but too dry and strong. (Makes you blackout after two -22oz bottles).
I would stick to the recipe, except dry-hop the blackberry puree, or a part of it. Once you've reached your desired ABV, hit it with Potassium Sorbate to stop the fermentation toss in your puree, let it sit a week or two, and rack it into secondary. Or rack and then dry hop. Or just go with your recipe, it's fun to go all mad scientist on it!