Black box unit testing focuses on testing the functionality of the application's individual components or units (each class and method) to ensure that they behave properly and satisfy a defined specification or use case. Having individual test cases for each unit of a code base allows for quick identification, isolation, and resolution of errors. Unit testing allows functionality problems to be identified and resolved as soon as they are introduced — which is easier and dramatically less costly than leaving them until the end of the development cycle. The practice of unit testing, when applied as a regular element of the software development process, has been proven to significantly improve application quality, reliability, and performance and to enhance the overall productivity and predictability of a software development effort.

Despite its well-perceived value, unit testing is not practiced by the majority of software development organizations. Even where it is practiced, it is typically applied inconsistently or incorrectly. The lack of general knowledge about how to write and apply unit test cases; compressed and overcommitted development schedules; the absence of good automated and integrated unit testing tools; and the absence of the appropriate guidance in how to apply them are all contributors to the slow adoption of the practice. Software development organizations trying to implement a unit testing practice without availing themselves of automated tools or the expertise in implementation and deployment most often fail.

The traditional practice of relying on QA to find and report errors at the end of the development cycle results in many unproductive man hours of development time spent sifting through large, complex code bases trying to find the source of those errors — all before a fix for the error can even be contemplated. Finding and fixing a unit-level error immediately after coding has proven to be 10 to 100 times less costly than finding and fixing the same error later in the development process. A practice of unit testing is key to achieving automated software error prevention.

When you perform functional testing at the unit level, you can quickly identify simple functionality problems because you are verifying the unit directly. If the same problem entered the shared code base and became part of a multi-million line application, it might surface only as strange behavior during application testing and go unaddressed. Implemented correctly, black box testing can be used to create test scenarios that mimic "bug" behavior. Test cases representative of inappropriate behavior or a specific bug will, when executed regularly, ensure that these bugs or incorrect behavior do not reenter the code base.

An effective black box unit testing practice requires knowing not only how to create test cases, but also how to apply and maintain them. In complex systems that often have multiple baselines, maintaining and applying black box test cases is challenging. Without a well-defined and implemented unit testing practice, inconsistent and/or inaccurate test results are likely.

To achieve effective black box unit testing, you must not only have a defined practice for its use, but that practice must be implemented and integrated into your software development lifecycle so that it is used consistently and regularly by your software development teams. It is also important to have the means to monitor and measure its use.