You have to use caution when blending herbs. For the most part they do not react, however they could.
So if you are not sure, try asking your local health food store where a herbalist will be able to help you and determine if there will be a reaction with the herbs.
Safe herbs, as found in our herb data base are generally safe to blend together.
If there is a side effect or caution listed, you really need a certified herbalist to assist you in your blending.