You can battle against Bianca using her team from the original Black or White version if you've connected your Black 2 or White 2 with your Black or White version using the Memory Link feature on the main menu (it's under Unova Link). The team she uses varies depending on which starter you chose in the original Black and White.
Assuming you've done the Memory Link with your Black or White version, as you walk in towards Nuvema Town, you'll be approached by Bianca, who wants to battle you!
Like Cheren, she can be battled only once. Her levels are even weaker than those of Cheren's, making her a relatively easy challenge. Just make sure you're healed before entering Nuvema Town if you've done a Memory Link.