A key difference between black box and white box test design techniques is that white box techniques can measure the extent of code coverage, while black box techniques cannot. Code coverage is a measure of how much of the code of a system or component has been executed by a test or a set of tests. White box techniques use software code to derive test cases that cover specific aspects of the code, such as statements, branches, paths, or data flows. White box techniques can use tools or methods to measure the code coverage achieved by the test cases and identify any gaps or redundancies. Black box techniques use functional design specifications to derive test cases that cover specific aspects of the functionality, behavior, or quality of the system or component, such as inputs, outputs, equivalence classes, boundary values, states, transitions, etc. Black box techniques do not use or access the code of the system or component, and therefore cannot measure the code coverage.
References: Certified Tester Foundation Level Syllabus, Section 4.3 and 4.4