Cloudflare SSL in a subdomain that is in another hosting

Hi,

I would like to buy an SSL Business plan in Cloudflare and configure it in example.com.

What happens if I create a subdomain called blog.example.com and is hosted on an external server targeted through Cname. Will I have SSL automatically? Or is it necessary to do the manual installation of SSL in the external hosting that has the subdomain blog.example.com?

It is. It always is. Cloudflare itself wont render a valid certificate on your server unnecessary.