According to Superherofix.com:
The episode, titled "Siren" also marks the return of Justin Hartley in his role as Oliver Queen/The Green Arrow. In DC Comics, the two vigilantes have a long and complex romantic history. However, when they first encounter on Smallville, they will be opponents. "When we meet her, she's a vigilante working for Lex Luthor," Gough says. The Justice League "has been keeping their eyes on Lex and trying to thwart him at every turn." Fed up, Lex "will say to Black Canary, if you can bring me the head of the Green Arrow, I'll make it very much worth your while."
Staying true to the comics, Smallville's Canary is a martial-arts expert who has a super-powered sonic scream, her "canary cry." Gough also confirms that she will be wearing the fishnet stockings that have become the character's trademark costume piece.
When asked if a romance would blossom between Green Arrow and Black Canary, Gough answered "It's the very beginning of their relationship. At the end of the episode you clearly see that they have some sparks." Undoubtedly, this will further complicate the relationship between Oliver Queen and Lois Lane (Erica Durance).
Hmmm... sounds interesting.
Wondering who will play the super songbird, Dianh? Alaina Huffman (aka Kalanj). You might know her from her recent roll as Maureen from Painkiller Jane. I think she looks too old/mature to fit into a cast of young adults, but we'll see. She probably is the same age, but I don't know that for sure.
Based on the episode list on Kryptonsite.com, we won't see this until after the show returns from it's long winter's nap.
-- jeeze, this is like my third TV show post in a row. And I call it a comic book blog - I'd better get something about comics on here pretty soon!