You said they are found in spinach and deli meats. I've been worrying ever since. I eat spinach in my salad every day. Also, a while ago, I read that it wasn't recommended to eat spinach on a regular basis. I apologize if what I wrote frightened you.
Spinach is great food.
The concern is when a nitrate turns into a nitrite and combines with protein to form nitrosoamine. With the deli meats, you have the nitrite and the protein present at the same time in the same food. With spinach, you have the nitrate, but you also have a number of helpful phytochemicals that make the conversion less likely. As an aside, I must admit that I am not a big proponent of the blood-type approach to dieting. If you are getting good results, that's great, but I have trouble with some of the underlying tenets of this approach.