It does not take blind faith to admit that the unknowable exists. What takes blind faith is trying to define the unknowable without having any way to KNOW it, and believing that your definition is reality. For example, imagine that there is a deep hole in the ground. It is dark; you cannot see the bottom. You don't know how far down it goes. You don't know what is inside, or if anything even IS inside. It does not take blind faith to recognize that the hole, the unknown, exists. However, if you start speculating about what is in that hole and reach a conclusion without having any way to find out if you are right, then your belief about what the hole contains--whether it be water, worms, bones, God, or the energizer bunny--requires blind faith.
Thus, it does not require blind faith to say that the Tao exists, as long as you don't go trying to define the Tao or say what positive characteristics it includes.