I see a patch of dark skin. Could this be something serious?

Dark skin. There's no way to tell you what's going on with the patch of dark skin which could be anything. There are many many other types of spots so you need to see a dermatologist!
Have it checked out. Could be post-inflammatory hyperpigmentation which commonly occurs in ethnic or darker skin or those who tan easily. It can occur after a rash or trauma or injury to the skin which then leaves behind dark patches. But you should see a dermatologist to make sure it's not anything more serious. There are some skin diseases which cause dark patches.