Edit: I use rclone as the backend for duplicity, so you can also chain it through another tool with different encryption and use rclone as just the transfer engine, getting all the benefits of rclone's providers with the benefits of duplicity's backup strategies.
If you're comfortable setting up a cron job, it's a great fit. I use it to back up a 1.5TB Samba directory and wind up paying about $5/m for B2.
You want tarsnap.[1]
Edit to add: Colin Percival is the author of scrypt[2] and has worked extensively with FreeBSD's portsnap, so he knows what he's doing.
To be more precise, I wrote FreeBSD's portsnap. (Also, freebsd-update.)
But one of the benefits of backblaze is the simplicity. Simplicity of setup, backups, and restores. If you muddle with that by encrypting before giving to backblaze you lose out on part of the value.
It also usually seems when people roll their own it opens up risk of forgetting something. BB is easy, set up their encryption and you should be fine.
Just some thoughts.
For restore, though, decryption happens at the server end. You have to supply your key to their server, which decrypts the data at their end, then sends you the subset you are interested in restoring.
See [1].
This is the only thing putting me off backblaze