How can blur a photo background on iPhone

You really don't need that app to blur the background when taking a picture with an iPhone. This is because, an iPhone 7 and above (newer iPhones) have had the ability to take pictures in Portrait mode. Portrait mode does exactly what you are looking for and that is it blurs the background when light and proper distance from the subject is maintained. Below is an example.

Image captured with an iPhone 12 Pro Max in portrait mode and then cropped to the square format. Image was shot in RAW format and converted to JPEG after a brief color adjust.

Axel F.

I see what you are referring to now. You would like the ability to pixelate a portion of the photo rather than hard mask it. As you already know, the built in Photos app on your Mac can mask just about any section in a photo. What it cannot do is pixelate a selection in a Photo. Of course, Adobe Photoshop will do it however that is an expensive proposition just for this one effect. A little bit of research led me to two apps, Snagit and Skitch for Mac.

Here is some detailed information on all of the above ➔ 4 Ways to Pixelate or Blur out part of an image on a Mac.

Axel F.