A blind tenon is created by tapering the cross grain mortise walls to create space for a wedge to expand a slice of the tenon into the tapered space, locking the joint in place mechanically. The joint has to fit perfectly because there is no retreat for adjustments. If the wedge is to big in any dimension, the joint will not seat properly. If it is too small, the lock will not be durable and there is no fix because even though it's loose it won't come apart.
The procedure is summarized in the following pictures. First, the router mill does what it does best. Then I square up the corners for a precise fit. You can barely see the tenon slice in the third picture. The fourth picture shows a square indicating how much taper to the bottom of the mortise. The fifth picture shows my technique for getting consistent taper by first cutting "marker" cuts so it is easy to see how much taper is developing. Then the larger mortise chisel is used to remove the remainder of the waste leaving a smooth and consistent wall.