Hi Kathleen! Kristen Eshleman sent me your way—maybe something along these lines might help?

RewriteEngine on
# if it doesn’t exist as a file or as a directory…
RewriteCond /path/to/docroot/%{REQUEST_FILENAME} !-f
RewriteCond /path/to/docroot/%{REQUEST_FILENAME} !-d

# …check whether it exists in /blog
RewriteCond /path/to/docroot/blog/%{REQUEST_FILENAME} -f

# and if so then redirect there and stop processing remaining rules
RewriteRule ^(.+) /path/to/docroot/blog/$1 [L]

# maybe some other pre-existing redirects in here?

# otherwise pass the request through unchanged
RewriteRule ^(.+) – [PT]