I changed the NS records in the registrar to point to Cloudflare. Then, on Cloudflare, I added ns1.sendinblue.com and ns2.sendinblue.com, but I never see the validation on “Check my DNS,” and I can’t validate it on BREVO.
If those NS records are entered into your Cloudflare DNS for your apex domain they won’t have any effect. You need to point at your host using A/AAAA/CNAME records, or if you need to set their nameservers at your registrar and not use Cloudflare.
You can delegate for subdomains using NS records in your Cloudflare DNS though.