Originally posted by Ira Although Lowell's answer is of course correct and the main reason why aperture blades are black, there is indeed another one which most people are unaware of:
It's an equal opportunity thing.
Ouch...Didn't expect that one from you Ira
As light comes through the lens, it is prone to reflect off surfaces. Those reflections cause ghosting and such, but also degrade the image relayed to the sensor. By putting a material that absorbs instead of reflects the light, that goes away as much as possible.
We don't see that as much in photography lenses as with telescopes, but the same exact situation exists. However, with telescopes, there is much less light, much greater light gathering capabilities, and a small area of missing paint inside the tube of a telescope is very easily seen. I had a Dob a few years ago with a small scratch on the inside that would do some horrible ghosting when looking at a full moon. It was due to this reason.