Redirect all /yyyy/mm/dd/ to /blog/

Sure, please see https://redirection.me/support/redirect-regular-expressions/

Thanks … I tried the following expression that I found on that webpage, but it’s not working so I must be entering it incorrectly.

Source: ^/\d{4}/\d{2}/\d{2}/(.*)
Target: /blog/$1/

For example, I want the following redirection to occur:
Source: https://www.ericsturgeon.com/2016/06/08/matterhorn-zermatt-switzerland-summer/
Target: https://www.ericsturgeon.com/blog/matterhorn-zermatt-switzerland-summer/

Using https://regex101.com/ I can see that your regular expression does match your URL. Have you enabled the regex checkbox? Have you cleared your browser cache?