True redaction vs black boxes
Most “fill a shape” tools, and a surprising number of online “redact PDF” pages, only drop a black annotation on top of the words. The text object is still in the file. A reader can select it, search it, or delete the annotation.
True redaction removes the content and, ideally, any leftover metadata. Adobe Acrobat Pro does this with a dedicated Redact tool. Blackout PDF takes a blunt, honest path: it paints the marks onto a rendered page and writes a new image-only PDF. There is no selectable text under the box, because there is no selectable text at all in the download. That is thorough for the words you marked, and it also means the rest of the page is no longer copyable either — a trade-off that is spelled out on the limits page.