Which direction of the normal vector that defines a Perceptron hyperplane is default?
This question is for general hyperplane. 

The normal direction is the direction that is perpendicular (also known as normal) to the plane. 

Take a piece of paper (treat it as a plane), hold a pencil perpendicular to the paper. The pencil is parallel to the normal direction. 

There is no such thing as the default direction as you see, a piece of paper that is oriented in a different manner would have a different normal direction.