Actually the front sight does block some light reaching the eye. The amount of blockage is the area of the front sight subtracted from the area of the objective lens. Probably a loss of around 20% of the light. . Since the human eye auto-adjusts for brightness it's not noticable at all. If the front sight were shiny it could scatter light into the field of view, but being black that is unlikely to be enough light to be noticable.
There is a bit of bending of light caused by the front sight. Diffraction effects will be present but usually too small to notice. In photos of stars you'll sometimes see "differaction spikes". Here's an extreme example:
In this photo of the star Sirius the four spikes are from light diffracted from the support vanes which hold the telescope's secondary mirror. There are in an equivalent positon to the front sight in front of a riflescope. It is not an image of the vanes themselves as they are completly out of the focal plane. If you looked at a distant bright point of light with the riflescope you could see simialar but weaker spikes s caused by the presence of the front sight. It would be a single thin white horizontal line with a little haze around it.