Okay, EC confirmed in Black Rock Shooter.
Spoilers below
Mato goes and fuses with her other world self(Black Rock Shooter), right as she kills her friend's other world self. This causes her to fall into despair and go insane which corrupts Black Rock Shooter, turning her eyes purple, and then transforming her into Insane Black Rock Shooter, who is psychotic and badass.
The transformation takes place within the first minute of episode 6 pretty much, pretty nice. Keep in mind that this link will only work till friday when the next episode will take it's place.
http://ch.nicovideo.jp/channel/blackrockshooter
And here's a nice before and after for prosperity