How to blur or censor a face in a photo
Use pixelation with a large block size rather than a blur. A Gaussian blur is a reversible mathematical operation and can sometimes be partially undone; pixelation averages whole blocks down to one value and genuinely destroys the detail. For anything sensitive, use blocks of at least 15 pixels, or cover the area with a solid shape.
Why blurring is the weaker choice
Blurring is a convolution: every output pixel is a weighted mix of its neighbours, and the original values still influence the result. Given the blur radius, deconvolution can recover a surprising amount, and faces are a case where even a partial recovery is enough to identify someone.
Pixelation replaces each block with a single averaged colour. The variation inside the block is gone and nothing can bring it back. The larger the block, the less survives.
Doing it properly
Crop to the region first if you only want part of the image affected, pixelate with a block size of 15 or more, then export. Check the result at full zoom — if facial structure is still readable, increase the block size.
For documents, addresses or card numbers, do not rely on obscuring at all: cover the area with a solid filled rectangle. It is the only method with no recovery path whatsoever.
A trap worth knowing
Redacting by drawing a black box in a PDF or a document editor often leaves the original text underneath, recoverable by copy-paste. Flattening to an image first, then covering the area, avoids that entirely.