Probably this is what most of us practice and is used most widely. This is also type of testing which is very close to customer experience. In this type of testing system is treated as close system and test engineer do not assume any thing about how system was created.
As a test engineer if you are performing black box test cases, one thing that you need to make sure is that you do not make any assumptions about the system based on your knowledge. Assumption created in our mind because of the system knowledge could harm testing effort and increase the chances of missing critical test cases.