PDF Redact
Not just black paint: you get a copy with the text under the boxes removed. Files never leave this browser.
Drag on a page to draw a box, or find text above. Click a box to remove it.
Covered pages are turned into images, so their whole text layer is gone. Safest, but those pages can't be searched or copied and the file gets bigger.
Out of scope: password-protected PDFs, PDFs over 60 pages, and finding text inside scanned images (cover those with a box by hand). The saved file is a newly built PDF, so bookmarks, attachments, form fields and JavaScript are left out, and comments on covered pages too. Pages without boxes are copied as they are.
The PDF engines (pdf.js, pdf-lib) load from this site, and your file is not stored anywhere.