Yeah, I ran the free trial too, but limited the folders to something I could upload and test during the 15 days! ;)

Regarding the question on security, all uploads are encrypted twice: The chunk of data modified since the last backup is encrypted on your machine using a highly secure protocol and an encryption key, then this encrypted file is sent to the server via encrypted HTTPS instead of clear text HTTP. The files you sent are just chunks containing only the bits of the files that were modified, not the whole files, so they would be harder to read and interpret even if a hacker had access to Backblaze servers. The key is not sent to the datacenter either, so nobody can decrypt your files. Furthermore, the files are not stored as files, the datacenter is not a drive and folder structure replicating your own computer, the whole datacenter is a gigantic SQL relational database where each chunk of your files are stored as binary records in a database table, which also require password security even to just read the files.

If your computer crashes, you need to provide your encryption key to Backblaze so that they can make the files available and you can download them for free or, for a fee, ask Backblaze to copy the files to a flash drive or a hard drive. That's the only time when a hacker working at Backblaze could have access to your files. You can probably limit your exposure by reuploading the same files using a different encryption key after you restored the files, or maybe using an encryption program so that specific critical files are always encrypted on your machine, which would probably result in the whole file being resent each time it is modified, not just the part that changed, it's a trade off.

Here is the FAQ on this topic. I think I read about their SQL database structure in their blog.

Don't forget that this is online BACKUP, not ARCHIVING, unlike some parts of Microsoft OneDrive or Google Drive, where once uploaded via the web site, your files are stored for life even if you delete them from your computer. In the case of an online backup service, if you delete a file on your computer, it gets deleted from the datacenter backup too. The files in the datacenter are an image of the files in your computer (including all external drives and maybe other networked computers that you added to your "desktop.")
There is however a grace period, which I think is 30 days, during which you can still retrieve the files before Backblaze deletes them permanently from the datacenter. It is by the way the same period and mechanism that allows you to retrieve any previous version of a file, for instance if it becomes corrupted. Every version older than this grace period gets automatically deleted from the datacenter.

Backblaze seems to be here for the long term. Even if they somehow fail because they were asleep for a year and did not notice that they were charging less than their costs, an IT company that has progressed so much in 7 years time, with over 32,000 hard drives representing 100 petabytes does not go bust, it is bought by a bigger one that will continue the business and make it profitable, which it currently is. For comparison, Facebook has only 4 times more storage. Backblaze added a new datacenter at a separate location last year, which will be able to store 500 petabytes, half an exabyte, or more when hard drive capacities increase.

I have been fascinated following their progression, from the first version of their home designed storage pods to the recently released fourth version. All plans and specs are in the public domain, so anybody can make the same storage pods, or buy an empty case from the metal works company that builds them for Backblaze, or even send the plans to your local metal works company so they can build one for you. Here are the blogs on the four different versions of "Petabytes on a budget":

1.0 - 67TB/$7,867 (1.5TB drives) - ¢11.7/GB
2.0 - 135TB/$7,384 (3TB drives) - ¢5.5/GB
3.0 - 135TB/$7,568 (3TB drives) - ¢5.6/GB
3.0 - 180TB/$10,718 (4TB drives) - ¢6.0/GB
4.0 - 180TB/$9,305 (4TB drives) - ¢5.2/GB
4.0 - 270TB/$28,722 (6TB drives) - ¢10.6/GB (The $695 6TB drives are not really available yet.)

I have all these links bookmarked of course!
As soon as the company goes public, I'll buy some of 'em stocks...
They seem to understand exactly what the consumer wants, and are high-tech smart too.