Well, be extremely careful with Backblaze. I got bit on the ass by them. It turns out their dashboard is just caching it's results, and there are instances where the dashboards will say they have data when they really don't.
I learned this the hard way when we brought my girlfriend's computer in for repair. We gave them permission to replace the drive, went to restore the data from Backblaze, and got a nasty surprise when it errored out and their support team let us know the data wasn't there. As far as I'm concerned using Backblaze is not the same as actually having off site backups, unfortunately.
That's not a real solution though, it's half of a solution at best. All you're providing is a place to dump files, with none of the services that a backup solution would actually provide.
This may be fantastic for technical users, but it would be a nightmare if I used this to manage the backups of friends and family.
Ultimately I switched to CrashPlan, as that not only gave me offsite backups (for far cheaper than you, sorry) but using the same technology also offers me on site backups and the ability to back up to any server I control and choose. This allows me to have my friends backup to the CrashPlan service, to their own local drive, and to a computer I setup just in case both of those fail. Best part is I'm not stuck doing a bunch of work.
As a final note, your service is extremely expensive compared to just about everything else.
Is this if I log into backblaze.com there may be files it says it has which it does not, or if the UI on my Mac says it's backed up it may not be? (Or both?)
I learned this the hard way when we brought my girlfriend's computer in for repair. We gave them permission to replace the drive, went to restore the data from Backblaze, and got a nasty surprise when it errored out and their support team let us know the data wasn't there. As far as I'm concerned using Backblaze is not the same as actually having off site backups, unfortunately.