blurring part of a photo

After blurring the first letter of the licence plate with the retouch brush:

You will have to repeat this more than once to make the letters vanish completely.

Can I blur part of a photo, in this case a license plate number, for privacy purposes?

You cannot exactly blur it, but you can use the retouch brush to make it unreadable.

Click the retouch tool in Edit mode, then option-click a spot with not much texture and drag across the licence plate.

You're welcome, Richard! 🙂