B2 claims to offer 99.9% uptime[1], and seems to have SLAs[2] similar to that of AWS S3[3].

[1]: https://www.backblaze.com/blog/cloud-storage-durability-vs-a...

[2]: https://www.backblaze.com/company/sla.html

[3]: https://aws.amazon.com/s3/sla/