How do I blur a face in a photo on iPhone?

The Photos app has no blur or pixelate tool. The built-in workaround is Markup, where you can cover a face with an opaque shape or a thick pen stroke. For a real blur or mosaic, use the free tool on this site in Safari, or the Anonimo app, which finds the faces for you and lets you resize each cover.

Markup in Photos: cover, not blur

Open the photo, tap Edit, then the Markup pen.
Tap the plus button and add a shape, fill it with a solid colour, and drag it over the face. Or use the pen at full opacity and scribble over the face.
Check the opacity slider is at 100%. A semi-transparent stroke leaves the face readable.

Markup covers rather than blurs, so the result is a block over the face. There is no mosaic, no emoji that follows the face, and nothing automatic.

The free tool on this site

Open the photo blur tool in Safari, choose the photo, and every face gets a cover: pixelation, blur or an emoji. Tap a face to keep it visible, add a cover by hand where one was missed, and download the result at full size. The photo stays on your phone; the page cannot upload it.

The Anonimo app

Anonimo opens JPEG, PNG and HEIC photos from your library, finds the faces, and lets you resize, reshape or drag each oval cover before saving back to Photos. It is the same app that does video; photos are free without the watermark.

Try the free photo blur in your browser

If you want this done automatically on your iPhone, Anonimo finds every face in a video, groups them by person, and blurs everyone except the people you pick. Nothing leaves the phone.

Trade-offs

Markup is built in and fast for one face; it cannot blur, and a thin stroke leaks.
The web tool needs no install and works on any device; it needs JavaScript and a modern browser.
The app handles HEIC natively and saves straight to Photos; it is iPhone only.

Questions

Can the iPhone Photos app blur faces?

No. It can cover them with Markup shapes or pen strokes, which hides but does not blur.

Is there a free face blur for photos?

Yes: the tool on this site is free, runs in your browser and adds no watermark. Photos in the Anonimo app are free too.

Does blurring remove the location from a photo?

Blurring alone does not. The web tool's download strips all metadata, including GPS. In Photos, you can remove the location from the share sheet's options.