Thanks for your question about learning to blend and mix teas.
It truly is an art!
The American Tea Masters does offer a course, you can visit their site here:
http://www.teamasters.org/index.html
But it is not directly related to 'blending'. There are so many things you can add to teas and herbal teas. Fruits, flowers, juice and many more items.
I think much of mastering tea blending is trial and error. You can also visit our herbal tea ingredients section for a list of herbs and the recipes for their basic herbal teas, along with medicinal uses.