Please add RewriteMap lc int:tolower at last of your htaccess file or in your <VirtualHost> section. Could you please try following then.
RewriteEngine On
RewriteCond https on
RewriteCond %{REQUEST_URI} ^/blog$ [NC]
RewriteCond %{QUERY_STRING} ^([^=]*)=([^+]*)\+(.*)$
RewriteRule ^(.*)$ /%1/${lc:%2}-${lc:%3} [QSD,R=301,L]
Testing with curl command(with http but rule above takes care of https part):
curl -IL "http://localhost:80/blog?category=Foo+Bar"
HTTP/1.1 301 Moved Permanently
Server: Apache/2.4.46 (Win64) OpenSSL/1.1.1g PHP/7.4.11
Location: http://localhost/category/foo-bar
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