Blood in baby poop?

My son has a suspected milk protein allergy, he has been on a few different formulas and is now on alimentum ready to feed. He started having blood and mucus in his poop again after about a week on alimentum. We took it to his doctor and they tested it, but said it was negative for blood even though we can clearly see it??

How can the test be negative for blood, what else could it be? My poor son is having bloody diarrhea and stomach pain at only 4 weeks old and I feel like they just arent doing their job right or caring.. ugh