No blur feature in iMovie!?

If you need a trackable blur you require a professional video editor.

Final Cut Pro is available as a 90 Day FREE TRIAL. It's completely. free as you don't have to give any personal details. Just click the download button and it's yours for 3 months after which it just stops working unless you decide to buy it.

Final Cut Pro for Mac - Free Trial - Apple (UK)

DaVinci Resolve has a completely free version which is every bit as good as FCP but which has a steeper learning curve.

https://www.blackmagicdesign.com/products/davinciresolve

There is a very simple workaround but the amount of blur cannot be adjusted.

Drop the Pull Focus title onto the clip itself (not above it) and delete the text in the Viewer window,

For feature requests, use feedback:

Product Feedback - Apple

To get this feature, I reluctantly had to use filmora.

Like I said, filmora will do it, too.

I suspect he wants a localized blur that follows an object, like a face, or license plate.

I believe you have to buy it otherwise it puts a watermark on exported videos?

Probably, I hated it for other reasons.