Which free tool can blur part of an image or video?

For a photo, use what you have: Google Photos offers portrait blur for backgrounds, and the Windows Photos app's Background panel blurs the background with an intensity slider, processed on your PC. For a video you post on YouTube, YouTube Studio's editor does it free on a computer: Face blur finds the faces and lets you choose which to blur, and Custom blur is a box you move, resize, time and set to track an object (a video with over 100,000 views may only allow face blur unless the channel is in the Partner Program). For hiding a phone number, account number, address or password, do not blur or pixelate it: security researchers at Bishop Fox showed pixelated text can be reconstructed, so cover it with a solid box using Markup's shapes on an iPhone or Preview's Redaction tool on a Mac. Rule: blur for faces and backgrounds, a solid block for anything that must stay secret.