Anemia would show up but not cancer. The blood test I have for cancer is a CEA. It's used once people are known to have colon/ bowel but NOT anal cancer. My CEA is very sensitive so a rise from 3.4 to 6.4 meant I had cancer on my liver. Some people are dx'd with colon cancer when their CEA's are 5,000. Normal is different at different labs but generally all normals are 5 and under. They don't do a CEA as a screening tool. Inflammation, benign breast conditions, smoking, polyps, constipation, age, weight can all cause false positives.
Pencil thin stools that stay that way are a symptom of rectal or anal cancer. Soft feces moves into the rectum and is squeezed thin when there is a tumor in the rectum or on the anus. Changes in bowel habits other than that or being completely unable to go are not a symptom.
BTW..... my FOBT cards were always negative. My husbands are always positive and he's had polyps (numerous) but no cancer. I'm 63 and had 1 polyp that WAS cancer. Go figure.