Why does blending blueberries, milk, banana and vanilla extract cause the mixture to have a yogurty consistency?
The pectin from the blueberries jelled in the presence of the calcium in the milk. The texture might be unexpected, but it is perfectly safe and tasty. It is the same process that thickens blueberry jam. 

It shouldn't be curdled, that is the clumping of milk proteins in the presence of acid. Here, the milk proteins stay suspended in the milk as usual, it is the pectin that builds the "mesh" of the creamy consistency.