I have read some about the Blood Type diet, but it is not a common type of diet or one that is easy to explain/understand.
I agree with the previous poster.
You would not be able to tell them you are type x and expect them to know what that means. You would need to be able to tell the chef/food service which food items or ingredients you need to avoid and which you are wanting to eat. That way,they can advise you on which food items on the menu meet your needs.
If you follow the link in my signature to the disABILITIES FAQs thread, post #3 of that thread has information about special diets and how to contact WDW to find out more information about special diets.