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

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.

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!