What %VG do you have? It could need a bit of a steep if it's under 2 days old, also I have heard rumour that if you use too high a percentage they can actually taste weaker, though I have no experience of this.
Yeah, should be fine if it's had a good shake, VG doesn't carry flavour as well as PG, so high VG mixes may need more flavouring. I'd be tempted to make another tester at 5% and see how that compares.
i have used FA blackberry at 4%. Blueberry may be weaker idk but it sounds to me that 12% maybe too strong.
+1 @scrumpox make sure you shake your concentrate before mixing
Fa flavors are very strong, so strong that 10% + will start to mute the flavor or it will just be sickly strong. Honestly, Make up a flavor at 2%, shake it good, and leave it 3-5 days then try it. FA are not the average flavor that you need 10-20% of just to get a half decent taste.