Ski boots are certainly not the most comfortable footwear, especially if they do not fit well. There are several reasons why black toenails can occur inside your ski boots. Some causes are more serious than others. Do not expect them to heal quickly either. The toenail must grow out to rid itself of discolouration.
Boots that are too small do not accommodate your feet and toes well. If one or more of your toes is constantly ramming into the end of the boot during your skiing, this kind of foot trauma will cause black toenails. A portion of the nail may be black or the entire nail. This is the result of bleeding under the nail.
Sometimes your boots fit well, but you may take some nasty spills and ram your toes into the end of your boot. This too can cause black toenails. Essentially, any trauma to the toes might result in a bruise underneath the toenail and then blackness results.
When the toes get too cold and lose circulation, frostbite can occur. This leads to a change in skin colour. Your toes and the associated nails can turn white, grey, purplish or black. Once you get frostbite, the affected areas are more susceptible again.
A black or discoloured area of your toenail might be a toenail melanoma. Wearing ski boots would not cause this, but if you recently went skiing and have a black toenail, you might assume it is from foot trauma or poorly-fitting boots. Always check with a medical professional to get a proper diagnosis of a discoloured toenail, especially if it has difficulty healing.