Episode 7x11 "Siren"
"While secretly working for Oliver (guest star Justin Hartley), Chloe (Allison Mack) intercepts one of Lex's (Michael Rosenbaum) project files, but she is attacked by the Black Canary (guest star Alaina Huffman), a mystery woman with a subsonic cry. Dinah Lance, Black Canary's alter ego, is a conservative talk show host who is working at the Daily Planet and clashes with Lois (Erica Durance). Lex convinces the Black Canary that the Green Arrow and his team are terrorists so she launches an attack on the Green Arrow and Clark (Tom Welling). Meanwhile, Lois discovers Oliver's secret."