Blake and Cinder met each other in volume 2 chapter 2 of rwby. In volume 3, chapter 12 Blake is seen glaring angrily at Cinder through the television when she reveals to everyone her true intentions. It has been noted that Cinder and Blake have many similarities. Firstly, they both had abusive pasts, Blake with Adam and Cinder with her stepmother. Secondly, they both have short black hair. Thirdly, they both have amber colored eyes. The two both also used to work with Adam Taurus at one point in their lives. In volume 2, chapter 7 the two are both seen wearing black dresses to the prom. Cinder and Blake also ended up working for villains when they were younger. Lastly, black seems to be both of their main theme colors.
In Adam's backstory, Blake was shown standing next to Adam when Cinder was offering herself to work with him. In volume 8 chapter 13, Cinder shows up infront of team rwby, Blake prepares herself to fight Cinder by taking her weapon out. Later when Neo "kills" Yang, Blake, devastated starts crying and Cinder is shown smirking in the background. Later when Cinder is giving another monologue Blake comes out of nowhere and angrily kicks her in the face, which sends Cinder flying backwards. Blake is then later shown fighting against Cinder alongside Penny and Weiss.
Black Glass is a rarepair in the rwby fandom, this is mainly because the two never "interacted" onscreen until volume 8. There were some fans that theorized that the two might have even been related since they looked so similar. There are even some fans who refer to them as the "disney princess pair" because the two are based off of Cinderella and Belle from Beauty and the Beast. Many fans were extremely satisfied when Blake kicked Cinder in the head in volume 8. It has a small fanbase on tumblr.
- Black Glass comes from the fact that Blake's name in old english means Black, and the fact that Cinder's semblance allows her to make black glass.