When I noticed blood in Alice's poos after I got her, she had all kinds of tests, including sending her poo off to be tested - The vet just said some cats have sensitive bots. Naturally, if it keeps happening or if he seems in pain, that is a different matter. From all the reading I did at the time, it said that blood in cats poo is rarely anything to worry about. In Alice's case, if she eats grass, as passes it, it is enough to make her bleed.