Hello, this is Dr K.. Answers are not medical advice and do not constitute a doctor/patient relationship. I look forward to helping you today.
It looks like you have been screened for many diseases.
Black or tarry stools means that there is likely blood in the stool. You need a stool test for occult blood. This is a simple in office test.
And you need a colonoscopy to look for Crohns disease or ulcerative colitis. Those would be fairly common diseases in your age group.
In the UK, you can self refer to a GI clinic I believe.