User blog comment:MetallicArcher/Fabulous Blake!/@comment-49.145.13.85-20140716160137/@comment-4820209-20140716165723

It's just a special effect when someone swings their weapon. Happened ALL the time in Vol. 1. Ruby had red slashes, Weiss had white, and Blake had bright purple. I forget about Yang however, but I'll assume she had none, as her weapon isn't bladed, and was usually shrouded in fire.