What would happen if our blind spot on the optic nerve had receptors?

You'd explode...

First off, there wouldn't be that much of a difference. Your nervous system compensates for the blind spot, anyways. Second, there's really no more room for any more photoreceptors there, because of the simple fact that the optic nerve originates there.

Evolution seems to have worked in the most efficient way, as the brain 'fills in' the missing spot it's just not necessary, if the spot did have receptors I don't think anything would be different.

A very small number of visual illusions would no longer work, but by its very nature you wouldn't notice. The only difference would be that you'd need to have these receptors shaped differently because of their position, and that would be a needless mutation.

If you suddenly had receptors on your blind spot, with no change in the shape of your eye, there would be a small fuzzy patch in your vision; that blind spot is not shaped to properly receive light focused by the lens. Far from improving your vision, you'd probably find it hampered slightly, and there would be a very minimal metabolic increase to support a few more cells.

As another said here: efficiency: no razor cuts more keenly than Occam's, and no less so when applied to evolutionary biology.