You cannot connect the Synology to your Mac via USB; there is no software on either end to make this connection work.
BackBlaze backup is not designed to allow backups of network shares. You may be able to “fool” BackBlaze by mounting Synology shares to non-standard locations as describe above, but I would be reluctant to rely on that approach in that it is possible for the BackBlaze software to detect such mount points and you might find your backups silently fail at some point. I always want my backup strategies to be reliable.
There is a way to connect the Synology so that it looks like a local disk, and that is to use SCSI over Ethernet. With both your Mac and the Synology on your LAN, you basically make some or all of the Synology storage look like a local drive, via an interface that looks like SCSI, but uses Ethernet as the underlying hardware rather than a (now obsolete) SCSI hardware interface and cable. This is not natively supported by MacOS, and requires software called “SCSI initiator” to be installed on the Mac. There are paid versions available, and I believe free versions as well. The Synology has native support fo this. However, this is probably NOT what you want. The Synology then behaves exactly like it was an external drive attached to the Mac; you format the drive using Disk Utility, and BackBlaze will see it as a local drive and back it up. However, the storage used in this way is NOT a Synology network available share, and cannot be accessed by any machines as a network share; it would be the same as a locally attached USB drive in concept. I don’t think this is what you are looking for.
The best solution I have seen for using BackBlaze backup for network shares is as another poster has mentioned: Attach a USB drive of sufficient size to the Mac, have Carbon Copy Cloner (or similar software) clone the Synology shares of interest to that US drive, and let BackBlaze back up the USB drive.
If you need a more “real time” backup of the Synology share, there are a few solutions. You can do the clone every hour, for instance, although there is a lot of overhead with doing this. You can use another Backup app like Arq, which will handle network shares, and have Arq backup the network share to the target of your choice (including BackBlaze B2, which is what I use), but bear in mind that you pay for storage used with this type of solution, rather than a flat fee for unlimited storage as with BackBlaze backup. You could use the Cloud Sync package on the Synology with one-way syncing of the share to a cloud solution (I use B2 here as well).
Hope this information is of use.