This is an untested WordPress plugin done as an example for a reddit answer found here: https://www.reddit.com/r/Wordpress/comments/absiu0/anyone_knows_how_to_make_an_image_blurry_until/
I wrote this in the GIST editor and have yet to test it on a WP Install. Please install at your own risk.
Download all files and upload in a single folder under /wp-content/plugins/
Within the wp-admin, activate the plugin.
[nsfw_blur] <img src="https://placekitten.com/640/480" alt="It's a cat!"> [/nsfw_blur]
Everything works except the blur. I really need something like this. HELP!