I've noticed that the nicotine MBV uses is unreasonably harsh. Also, some flavorings tend to be harsher than others. I think MBV uses FW flavorings, and personally I haven't tried their blueberry, but maybe someone else has and can speak on it's harshness.
I have some Blue Moo that is 50/50 and 0mg nic, and it isn't harsh at all. So I would venture a guess that it is the nicotine.