, welcome to Just Answer. I will do my best to answer your questions and help Kandinsky. Did he vomit the blood or did he pass it in feces?
Okay, that does help. How is he acting now? Any difficulty breathing? Does he have blood around his hindend or on the tail?
Without doing an actual examination it will be difficult to tell you exactly where the blood came from and if there is anything wrong with him. Having said that, I can tell you that when cats eat grass and play outside they can have a blade of grass stick in the soft palate and upper sinus areas at the back of the mouth. This can cause significant irritation until it is expelled. Some cats will cough or vomit to rid the blade of grass, and in doing so will rupture some of the blood vessels in the area, which leads to a puddle of blood being brought up as well.
It is possible that this is what happened to Kandinsky. If he is comfortable, I think it is reasonable to watch him and monitor his appetite and bathroom behavior. If they appear abnormal, or he seems weak, nauseous, or you see more blood expelled, then I would have your veterinarian examine him.
Watch coughing, gagging, drooling, pawing at the mouth, vomiting, difficulty breathing, or frequent swallowing. These could all be signs of irritation and even a foreign object.
My pleasure, good luck!