The blind spot occurs due to the structure of the eye.
When light travels reaches the back of the eye, it's wavelength is interpreted by the group of cells collectively called the retina.
In the middle of the retina is an area called the fovea, a place where many blood vessels are located. As there are no cells that detect light within the fovea, there is a natural 'blind spot' created in your field of vision. Luckily your eye moves up to 3 times a second, adjusting for this blind spot!