No. The gold standard test for bowel cancer or other inflammatory bowel issues is a colonoscopy. It's the only way they can really tell.  Bloods are fine for markers if you've been diagnosed with something, if that something gives off markers, but that's about it other than being anaemic. But not everyone with bowel cancer is anaemic nor have markers present. Blood tests can suggest something is awry sometimes, but not always.

Even the cancer markers don't mean much on their own. Those markers can be raised for other reasons as can being anaemic.

Read on her the other week that it tends to go like this,  dark blood (maroon) is usually from your right, black/coffee granule colour from your stomach and red from your left sided colon to your anus. Food colouring can also give you a bloody looking stool. I once ate one of those red cakes and i was at the doctors the next day. Turned out to be that cake.

I would ask for a colonoscopy. Sounds gross, but if the doc isn't seeing the blood, you might need to take a picture so they can see it for themselves.