I believe in this case Cloudflare and Backblaze would be considered service providers, not network providers (like Comcast or AT&T, etc...). And you'd still need to pay each service individually for the service they're providing. It's an alternative to say, Amazon S3 and their own CDN which also does not charge for egress in between the two services (since they own both). But in our case, you pay Backblaze B2 Cloud Storage of the storage, and you pay Cloudflare for their CDN capabilities, the partnership simply makes the transfer between our storage and their CDN free.

Transit does cost different amounts of money depending on who you negotiate transit with. That's not part of net neutrality.