As seen on Myanimelist.net:

Black★Rock Shooter tells of the narrator’s despair and hopelessness as they wander aimlessly, searching for something important to them that they presumably lost. Eventually the narrator gains new hope and decides to move on into the future with courage. The video for the song became massively popular, featuring Black★Rock Shooter traveling through a bleak city and countryside, riding a motorbike and wielding weapons. Black★Rock Shooter is an original character design by Huke, and happens to resemble Miku.

A famous 3DPV by Anomaro-P gave Black★Rock Shooter a plotline and backstory, which involves her sister betraying her and turning evil and Black★Rock Shooter being forced to kill her sister to stop the destruction. This plot eventually became the inspiration for the upcoming 2010 anime.

Vocaloid related project scheduled for next spring. Preview is planned for September 30, release of the anime in April 2010.

Related posts:

1. Black Rock Shooter: Black Blade Edition
2. Black★Rock Shooter The Animation – First Impressions or is it?
3. Anime of 2010 Q2
4. Black Mirage Theme
5. Hatsune Miku: Project DIVA – PSP