Esophagitis,gastritis,gingivitis,ulcers in the mouth, recent dental work are common gastrointestinal causes of blood in saliva.Bronchitis,nosebleed,
pneumonia,pneumothorax are respiratory problems presenting with
blood in saliva. Other rare but serious causes are esophageal cancer, lung cancer, stomach cancer etc.What are the other symptoms you have been having? A clinical examination and work up like X-ray chest, blood counts and endoscopy may be needed. I sincerely hope it helps.