According to EW, Arrow actress Katie Cassidy will guest star in the second-to-last season 2 episode of The Flash as an alternate version of Laurel Lance/Black Canary. The news was confirmed today by Arrow EP Marc Guggenheim during a press screening.
Apparently this Earth-2 incarnation of Lance will be named Black Siren, who is actually a character from the Justice League animated series that was based on the golden age version of Black Canary. Cassidy will also voice Earth-1 Laurel on a season 2 episode of the Vixen animated series.
Thanks to Twitter user Diva_Jong we have a look at Cassidy shooting a scene as Siren for The Flash.