why is your vision blurred when we view through a blotting paper

This is because blotting paper is translucent and allows only some light rays reflected by surroundings to pass through  it , thus your vision gets blured ...you even see blurred vision in many diseases because of the way light rays reflected from the surroundings reach you due to defect in mechanism of your eye

Because it allows only some light rays to pass through it which are reflected by the surrounding.It is the principal cause.