Brian from Backblaze here.
We support Linux now with our command line tool found here: https://www.backblaze.com/b2/docs/quick_command_line.html There is no GUI, but the command line tool can "sync" your files with an arbitrary roll back time (the Mac/Windows client are hard coded to 30 days).
For good or bad, syncing to B2 charges "per GByte". If you have less than 1 TByte this will be cheaper than the $5/month for the Mac or Windows client. But if you have more than 1 TByte on Linux, it will cost you about $5/TByte/Month to back it up.
They say they don't have enough workforce for the Linux client to happen now, not that they won't ever do it.
https://help.backblaze.com/hc/en-us/articles/217664628-Is-Ba...
Spoiler alert, the answer is a surprisingly firm no. Not even an optimistic "we're working on it!".
Doesn't even have to be "Linux support", just open an API, we'll do the rest! Use quotas for API users if the concern is around abuse, but please help us Linux folks use your service! Please!