There was a reason for the mirror not to see through Rean's concealment ability. It was the fact that what hid the Dragon Wing Marks was not the Light bending skill itself but the light it had bent. There was absolutely nothing wrong with the light reflecting the blue color of the marks other than the fact that it was forced to follow a pre-determined path. Rean's concealment ability against naked eyes was near impeccable because of it. Unlike other skills that concealed things themselves, Rean's skills forced something else to do the job. In this case, it was the light.
Since Celso thought Rean's group didn't have the Dragon Wing Marks, they preferred not to attack. After all, they believed Rean's group would die to the next Light Element and Divine Energy wave. It would be unwise to attack them in front of everyone and alert the rest of the cultivators who had nothing to do with the Lost Star Selection.
