Board Thread:Speculation House/@comment-64.113.177.152-20150121173112/@comment-4010415-20150316012036

It is suspected that the ability could be exclusive to Sun, as the original Sun Wukong has the ability to see through disguises, and we haven't seen any evidence of other Faunus being able to detect fellow Faunus.

In a nutshell: Sun saw through Blake's "disguise," probably just as a reference to the original Sun Wukong's ability to see through disguises.