I went through a colon cancer scare back in October. I had basically the same symptoms as you, plus sudden onset of diarrhea. Not fun. I had no appetite, and actually lost about 14lbs in 7 weeks! My stools were NOT what I was used to, and even had undigested food in there. I too saw black in my stool - come to find out it was undigested salad. Yuck!
I had a colonoscopy done and they never found out what exactly it was. My gastro thinks its post-infectious IBS-D. I was put on a medicine for IBS-D and it cleared everything up.
I attributed the loss of appetite, HORRIBLE fatigue and weight loss to SEVERE anxiety. I wasn't having panic attacks, but I wasn't eating or taking care of myself as I should've been. I know how hard it can be to do so when you're so consumed with fear. And anxiety and stress takes a huge toll on our bodies. I believe that I was severely depressed (who wouldn't be when they think they're dying?) and that also leads to loss of appetite, fatigue and weight loss.
Please eat something. I pretty much lived on Ensure shakes and fruit. I'm still not 100% convinced that there's not something pathological going on, but I feel better since my doctor does.
Go see a doc and talk to him/her about it. At least it will put your mind at ease. Can I ask how old you are? I'm 26 years old, and when all of this started happening I was SURE I had colon cancer. And when I looked up the statistics - people under age 39 have a 0.08% chance of having colon cancer - very VERY low. I hope that reassures you. Also - another thing to think about is that most people who actually DO have colon cancer have blood in their stool. From the symptoms you've described, it would have to be very very advanced colon cancer to come up with those symptoms. And there is no way that very very advanced colon cancer WOULD NOT show up with blood in your stool. It's just not possible.
Hope I've helped!