Is it unusual to have a bruised abdomen after a colonoscopy?

Yes: Occasionally manual presure on the abdomen is needed to overcome coiling of the scope. This may cause some soreness afterward. Bruising may occur if you are on a blood thinner. However it is pretty unusual to see bruising after a colonoscopy.

Possible: But not the colonoscopy per se but rather manual compression of the abdomen is sometimes needed to guide the scope to the full extent of the colon especially with a tortuous colon or prior abdominal surgeries. But even with that you may be sore but should not have any visible bruising.