This is a wonderfully healing and mineral rich smoothie. Don't be afraid of the comfrey! It is very safe to eat and use every day (yes, internally!). It is one of the best healing herbs that there is, but it has gotten a bad rap. If you are still hesitant, then just leave it out.
Here's a very informative and interesting article on the use of comfrey: http://www.herballegacy.com/ThesisContraIndications.html