Backblaze doesn't allow you to backup iCloud files, unless you make a copy on your HD. I tried to cancel. Their cancellation process is horrendous and possibly illegal (the FTC now requires click-to-cancel, and forbids automatic renewal without consent). With BackBlaze, if you cancel before your paid subscription ends, you just lose your remaining time. To cancel, you have to delete all your data on their site, and then "delete your computer". (You should also wait until your subscription is about to expire, so as not to lose the time you've paid for).
For safety, I would advise blocking them from your credit card, if possible.