Is that a service or you mean “blog” as WordPress Website?
Are you using a CNAME record if this is a service?
What is your domain name?
May I ask, before moving to Cloudflare, have you had an valid SSL certificate installed at your host/origin server for your Website, both covering non-www and www domain?
Was the Website working fine over HTTPS before moving to Cloudflare?
Kindly, may I ask have you checked within the steps from below articles regarding this error showing up for your Website?:
Check SSL/TLS tab at Cloudflare dashboard and make sure you setup and have the Full (Strict) SSL:
Here is a way to re-check if you correctly setup the SSL for your domain with Cloudflare:
If any other issues appear, follow the needed steps for troubleshooting from article below:
Regarding available SSL options at Cloudflare dashboard, check here:
In case you do not have an SSL certificate, you can use Cloudflare SSL, if so, kindly make sure you follow the instructions as follows on the below article to setup an SSL certificate using Cloudflare CA Origin Certificate: