Inking of peach is the appearance of dark blue to almost black superficial spots on the fruit surface. According to Dr. Carlos Crisosto (UC Davis), who has probably done the most work on this issue, the spots are caused by a combination of abbrasion and heavy metal contamination (Crisosto et al. 1999; pdf). Some heavy metals in combination with abbrasion (including the disturbance of the peach fuzz) will destroy peach cells of the peach skin containing pigments such as anthocyanines. That is what causes the dark color. His research shows that especially aluminum, copper and iron are culprits of inking. They can come vrom a variety of souces like dust (fine dirt particles that drift onto trees) overhead irrigation, hydrocooler water, fungicides, insecticides, etc. Contrary to the intuitive thought that water would wash off the metals, rainfall or overhead irrigation may increase inking, as it washes metals, dust, etc. to the fruit from the leaves. Iron (Fe) levels above 250 ppm in water will cause direct inking, but you will not likely see Fe levels this high in well water. With abrasion, Fe levels >10 ppm will cause inking. Well and hydrocooler water should be checked as part of any investigation of inking. The levels are not researched as well, but levels > 50 ppm for Aluminum (Al) and Copper (Cu) will also cause inking when abrasion is added.
Here are some commercially available materials that contain unusually high levels of inking-causing heavy metals (for a more detailed list of heavy metal contents in Ag products click here):
Foliar nutrients: Micro Plex (Fe)
Insecticides: Imidan (Al), Delegate (Al)
Miticides: Vendex (Fe and Al), Acramite (Fe and Al), Omite (Al)
Fungicides: Elite (AL)
Keep in mind that the availability of heavy metals is dependent on the pH of the spray solution, the type and concentration of ligands on which the metal could adsorb, and the oxidation stage of the heavy metal. In general, the lower the pH, the more available are heavy metals. Try to keep your spray water at a neutral pH and you should be fine.
Tips to reduce inking: