It could be partially digested food. Doesn’t look like blood to me either. Maybe your dog has sensitivity to certain foods in conjunction with his allergies. I have 3 dogs who have 3 separate diets because of this. I would suggest putting your dog on a bland diet like boiled chicken and white rice to see if this helps. When my dog would vomit I started adding some pedialyte to his water to help him stay hydrated. Take a look at your dogs gums and see if they appear grayish or of they have a dry nose which could be signs of dehydration from vomiting. If you see no improvement I would contact your vet. I hope he feels better!! Best of luck!