The only health care related item that the attorney's permit us to provide in documents is the naming of individuals you wish to make health care decisions in the event you cannot do so for yourself.
Specific or detailed instructions to your health care provider is done in the Health Care Directive. Instructing your physician that you want do not want a blood transfusion is a medical issue, the implications of which are not legal in nature. Such instructions are a health issue and are treated like any other direction to your health care providers in your Health Care Directive. You should consult with your primary care giver and then manually add your wishes about blood transfusions (and any other specific instructions - on hydration, pain medication, organ donation, etc) to your Health Care Directive.
In addition, you can make sure you provide verbal and written instructions to the individuals you empowered to make health care decisions on your behalf so your health care agents can make sure that your wishes are carried out.