Blueberries can definitely give low pH issues in some batches, and because they contain some organic acids like benzoic, they can be hard to ferment at times. I'd check the pH and adjust with some potassium bicarb if it is less than 3.2 with the goal of getting to around 3.5-3.6.
The reason things didn't get going when you tossed in a starter is that you tossed it into an environment that was hostile with around 7% ABV. Unless you acclimate your starter prior to pitching, and alcohol level that high will stun most of the yeast you pitch in, especially if the pH is already a factor.
If you find the pH is not a problem, I'd treat with 1 gram per gallon of yeast hulls to bind toxins and then I'd repitch using an acclimated starter (see Hightest's directions in the sticky at the top of the forum) using a yeast that is good for restarts (Uvaferm 43 would be best, but EC-1118, DV-10, or Premier Cuvee will usually do).