The presence of blood in the puppys stool after deworming is normal. Blood is what you may see in the feces because the parasites were feeding on it. During the process of killing and destroying them in the intestines blood is expelled into the feces and not absorbed. The deworming process should not cause concerns unless you notice a lot of blood in the feces which might indicate another problem. Pay attention to the blood color. Bleeding in the lower intestines might be indicated by bright redness and you should see a veterinarian.
Remember that after deworming your dog he should not have blood in the stool for longer than two days. Consult your veterinarian if the problem persists.