Short answer
Visual black-box redaction is not always safe because sensitive text may remain in the underlying file layer, metadata, annotations, or OCR output.
Expanded answer
A black rectangle on a PDF page may hide information from human view, but it does not always remove the original data from the file.
If the underlying text layer remains, users or software may be able to copy, search, extract, or recover the hidden content. Safer redaction should permanently remove sensitive information from the document rather than only covering it visually.