Why change puppy's food? Update on blood in puppy's poo.[From 6:30 this morning]?

Update on this morning's post:No worms, no ear infection,no parasites.Possibly something he ingested.Vet put him on canned I/D for 4 days.Gave me spiel about Science Diet food,30 years research backing it up.I feed Blue Buffalo dry puppy formula.Also did express,another dose de-worming.We will just have to see how he is by Monday.Thanks for the help while waiting for vet to open.Puppy still playing and alert.