Are you taking any kind of medications? Are your forearms more exposed to the sun than the rest of you? By "black," do you mean the color of black ink, or do you mean darker than normal? Apparently taking too much iron can cause skin to darken, and there are other causes as well. Here's a link to an article about skin getting either lighter or darker than normal:
https://www.mountsinai.org/health-library/symptoms/skin-abnormally-dark-or-light