I have colostomy bag. Recently had some blood in stool. Does doctor have 2 wait till blood is absent to give me reversal operation to remove bag?

Probably not. Obvious gross bleeding would delay things until cause of bleeding is found. Minor bleeding from edge of stoma or stoma itself should not be of concern, nor should test for "occult blood" which is usually positive in colostomy patients.
Not necessarily. It would depend on the source of the bleeding, and why the colostomy was needed. Your surgeon should know about the blood, and may want to investigate it, but there can even be minor bleeding from irritation of the stoma from the ostomy appliance.

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