Redirect old posts now '/blog' has been added to permalink

My blog posts used to have the permalink:

sitename.com/%postname%

But I’ve now changed the permalink to:

sitename.com/blog/%postname%

This all works fine to navigate around the site, but I’m worried about old links on other sites that point to blog posts on my site. So I’m now trying (and failing) to set up a 401 redirect to take care of this.

Anyone know how to do this? Thanks.

Addition:
I forgot to mention, my WordPress install is in a subfolder, not at the root. Not sure if that makes a difference or not.