The doctrine
Test from the visible inputs and outputs, assume nothing inside.
Black-box testing means auditing a target workbook through what it takes in and what it puts out, without assuming the internal formulas are correct — and the guide states the reason directly: it prevents a false sense of security from formulas that calculate but produce wrong operating behavior. The distinction matters because Excel is exceptionally good at producing plausible wrong answers. A formula referencing a stale range, a lookup keyed on a floating-point value, a rollup silently including future blank rows: all of these compute cleanly, display convincingly, and are wrong. Only a controlled input with a pre-stated expected output catches them.

