Joey Styles has posted a new article over at WWE.com, in which he explains the real life past that both Sin Cara wrestlers have with each other.
The story notes that the black Sin Cara used to wrestle as Mistico in Mexico, however, CMLL trademarked the name Mistico, and later gave it to the wrestler who plays the blue Sin Cara. So it is in fact true that when the black Sin Cara refers to the blue Sin Cara "stealing" his persona in the past, that it did actually take place in Mexico. You can read the entire article by clicking on the above link.