The phrase "black backpack" refers to a backpack that is colored black, which means it is completely or mostly dark in color.
Sentences with «black backpack»
If I want to dress it a little more casual, I would switch the faux jacket with a leather jacket, put on some white sneakers and a cute black backpack. (teacupsandtulips.com)
Add a pair of brown high ankle shoes and a cool black backpack to complement the rugged look. (myntra.com)
She keeps the denial letters in a little black backpack. (wamc.org)