Your dog could have some parasites or other worms eating the dogs intestines which could be causing blood in your dogs stools.
Other things are the dog could've ate something red such as a red crayon or it could be something more serious.
Most commonly though the cause of blood in a dogs stool is due to worms in your dogs intestines which need killed and treated before the worms eventually kill your dog.
You should take your dog to the vet to have a stool sample taken so they can find out what is actually causing the blood in the stool.
The blood in your dogs stool is not normal and shouldn't be ignored or it could potentially kill your dog so get the dog to the vet to get checked out and you may have to get the dog on some worm medicine.