Warning: major spoilers ahead for Episode 11 of Arrow Season 5.
Arrow has been without a Black Canary ever since Laurel Lance was murdered at the end of Season 4, but it was only a matter of time before Arrow took the chance to introduce a new version of the character. Oliver finally decided that his team of newbies was incomplete without a Black Canary, and he spent most of "Second Chances" trying to recruit a former cop named Tina to take on the mantle. Tina finally agreed to fight the good fight in Star City, and she then dropped a big DC Comics bombshell on Oliver. "Tina Bowland" is only the name she used as an undercover cop; her real name is Dinah Drake.
Dinah Drake is the name of the very first Black Canary of comic canon. The character fought crime as a vigilante and worked her way up the superhero ranks until she became a member of the Justice Society. She also married Detective Larry Lance, and they went on to have a daughter that they named Dinah Laurel. In comic canon, Dinah would eventually pass on the Black Canary persona to her daughter, and Dinah Laurel would become the second Black Canary.
Although Arrow has borrowed names from comic canon plenty of times over the years, Dinah Drake is still a crazy surprise. Many fans assumed that Laurel and Sara's mother was Dinah Drake, and she'd simply never become a vigilante in the Arrow universe. Dinah isn't exactly the most common name in the world, and it seemed likely that Laurel's mom's maiden name was Drake. The Arrow crew managed to pull a pretty epic "Gotcha!" with the big reveal that Oliver happened to stumble on another vigilante called Dinah. It was a brazen move from a show that had to find a new way to introduce a fourth version of a character, and it's sure to have people talking.
In hindsight, it actually makes sense that Black Canary 4.0 is named Dinah. In her dying words, Laurel asked Oliver to promise that he would find another Canary so that a part of her "will always be out there" with him. I don't think any of us - or Oliver, for that matter - would have guessed that her replacement would literally bring a part of Laurel to Team Arrow. Dinahs aren't exactly a dime a dozen, and no matter how long she sticks around, Dinah Drake will be a constant reminder of what Laurel brought to Arrow.
We'll have to wait and see how important Dinah Drake is to Oliver in the coming weeks. The Black Canary and Green Arrow of DC Comics canon have been romantically linked off and on for decades, but Arrow has already hooked Oliver up with two Canarys. Third time isn't necessarily the charm when it comes to selecting bedmates. We may see Oliver and Dinah establish a healthy working relationship without any unnecessary complications. He already has plenty of those with Susan Williams.