Also, it uses to happen because the copper lines of the "ribbon board" wears out
When is new that copper lines should be very shiny... but when the time passes it happens some kind of oxidation in the copper and goes darker, in this photo can be seen, the 2 copper lines most at the right are starting to darken
Usually after some years of use of a dualshock3 you are going to have that lines very dark (not only a couple of them, but all of them), is normal
When are so dark electrically is still workinng, but the connections are bad
If you try to clean them up with warm water + soap the dark is not going to be removed
If you try with alcohol what happens is the whole copper trace is going to dissapear (so you have broken that plastic ribbon circuit)
There is no way to return the shiny copper look like when it was new... the copper has been oxidized (or some chemical reaction at microscopy level)
If you want to buy a replacement are around 3€ free shipping or so, are not expensive
But be sure you are buying one that is compatible with your dualshock3 model (by comparing the old one with the new), there are several models and have small differences, you can see a list of them here
http://www.psdevwiki.com/ps3/DualShock_3#Ribbon_Circuit_Boards