This is what I found on it:
Reasons Potatoes Turn Black After Cooking:
Potatoes occasionally turn gray or dark after they are boiled; this color change may be caused by the conditions under which they were grown or stored. It's impossible to tell which potatoes will turn dark, but the discoloration does not affect flavor, texture, or nutritional value. Contact with aluminum or iron will also discolor potatoes, so cook them in stainless steel pots.
More reasons why cooked potatoes turn black:
According to the Idaho Potato Growers at http://www.idahopotato.com/faq/index.php:
If the potatoes were stored too cold (below 40 degrees F) then they can turn black. Could also have black bruising from being handled (dropped) when cold. If your potatoes turn black after they are boiled, you may be using an aluminum or reactive pot, so it's important to use a pot that's non-reactive.
While cooling, iron in the potato combines with other natural compounds, causing a grey, black, or bluish purple color. To prevent this, after the potatoes are cooked and drained, stir in a small amount of lemon juice and keep them covered with a tight-fitting lid.
According to the American Journal of Potato Research:
After-cooking darkening (ACD) is one of the most widespread, undesirable characteristics of cultivated potato. With the current expansion of the potato-processing industry around the world, there is a renewed interest in the development of new ways to prevent ACD. After-cooking darkening is caused by the oxidation of the ferri-chlorogenic acid in the boiled or fried potatoes.
The severity of the darkening is dependent on the ratio of chlorogenic acid to citric acid concentrations in the potato tubers. Higher ratio normally results in darker tubers. The concentration of the chlorogenic and citric acids is genetically controlled and influenced by environmental conditions.
This paper outlines the history of ACD and current status of knowledge of the chemistry of the dark pigment formation and its genetic and environmental determinants. Also discussed are the methods of chemical prevention of ACD presently used by the potato-processing industry and potential strategies for reducing tuber after cooking darkening using molecular approaches.