When testers don’t know anything about how the software has been put together, they are black box testers.
Their lack of knowledge of the internal structure doesn’t mean that black box testers are completely in the dark. These kinds of testers should get a document or a brief that explains what software is supposed to do. In the case of black box functional testing and test cases, the brief should cover what specific functions or usages the testers should try.
Black box testing is more open-ended, since testers don’t know the internal details of the system or what the developers expect users to do. Since they’re coming to the software without any preconceptions, black box testers can discover a completely different set of bugs and issues.