"A specification-based test that looks at a system or unit exclusively from the outside, that is, over its public interface." (Johannes Link & Peter Fröhlich, "Unit Testing in Java", 2003)
"This test compares the externally observable behavior at the external software interfaces (without knowledge of their structure) with the desired behavior. Black-Box tests are frequently equated with »functional tests«, although they can of course also include non-functional tests." (Lars Dittmann et al, "Automotive SPICE in Practice", 2008)
"Repeatable procedure to derive and/or select test cases based on an analysis of the specification, either functional or nonfunctional, of a component or system without reference to its internal structure." (Tilo Linz et al, "Software Testing Foundations" 4th Ed., 2014)
"A software testing methodology that looks at available inputs for an application and the expected outputs from each input." (Mike Harwood, "Internet Security: How to Defend Against Attackers on the Web" 2nd Ed., 2015)
[Data coverage (black-box) testing:] "Testing a program or subprogram based on the possible input values, treating the code as a black box" (Nell Dale & John Lewis, "Computer Science Illuminated" 6th Ed., 2015)
[black-box test design technique:] "Procedure to derive and/or select test cases based on an analysis of the specification, either functional or non-functional, of a component or system without reference to its internal structure." (Software Quality Assurance)
"Testing, either functional or non-functional, without reference to the internal structure of the component or system." (Software Quality Assurance)