It's rather easy to get TOS'ed even if you match filenames of common pirate name signatures.
Do you have any examples where someone's Backblaze account was terminated for backing up pirated media?
I still suggest it for friends and family because it's cheap and damn sure that encryption is better than nothing, but if you have a lot of sensitive data I wouldn't recommend it. If you demand privacy, use Restic and Backblaze's B2 service, you're paying per GB then though.
First, though, find out if your friends or family have an Office365 subscription. A lot of people get one because they want the Office apps, and don't make heavy use of the 1 TB of OneDrive storage that it includes. Restic vis rclone should be able to use that for backups.
Backblaze is cheap, but using something you have already paid for is even cheaper. :-)
Several other cloud backup programs do have OneDrive support built in, including Arq, duplicity, duplicati, duplicacy, and GoodSync. There are probably others.