There are a few things that can cause 'dark skin'. The definition of dark skin can sometimes be mistaken for blue skin. Blue skin is caused by cyanosis and poor circulation. As we get older the heart doesn't have as much strength as it once did and the extremities often don't get enough circulation. This condition can be serious and needs to be examined by a doctor. Some other very common signs of blue skin are: Varicose veins, broken capillaries, bulging veins and bruising. These are very common in the elderly due to thinning skin, weakened and fragile blood vessel walls, and difficulty with clotting. Medications such as blood thinners are often prescribed to the elderly and this can make the already impaired vessels even weaker.
A condition known as hyperpigmentation is caused by some serious illnesses such as Addison's disease. Dark skin is caused by certain cells releasing to much pigmentation. This also needs to be addresses by a health care professional as soon as possible.
There are several other conditions that can cause dark skin, blue skin, red skin, purple skin, hard skin, and black patches of skin. I am attaching a web-site for you to read through.
At 87 years old it would not be uncommon at all to have several conditions at the same time. With thinner skin and poor circulation you may vividly see the blue blood vessels of the extremities. The most important thing to remember is that unless you have had this for a long time and your doctor is already aware of it then you will need an evaluation as soon as possible. Once your doctor figures out the cause they will determine if it is something that treatment would be an option.
If the dark skin comes on rapidly and there was no injury to the area and it is accompanied by any other problem such as shortness of breath, chest pain, abdominal pain, dizziness, weakness or any other symptom then you need to call for an ambulance. Sudden onset of dark skin in the extremities can be a sign of cardiogenic shock. If it has progressed slowly over time then you can call your physician as soon as the office opens.
The above web-site goes into more detail of the causes of dark skin and if you read through carefully you should be able to compare your symptoms to the description.
If you need anymore assistance please let me know. I hope this information was useful.