Time Machine is a disk image split into bands. When you create a new backup new data is written and it may result in multiple bands added and/or modified. Then when you sync each band needs to be uploaded in its entirety, even if only a few bytes changed. If your Time Machine is encrypted — the data spread is even more amplified.
The advice is to not sync Time Machine to Backblaze. Time Machine is a local backup. It contains a lot of data that is nice to have if bandwidth and storage is not a concern but can be 100% omitted from offsite backup. Stuff like derivative data in your photos library and browser databases.
Instead, use another tool to backup your actual source data (without derivative; using photos library again as an example — pick only originals folder) to Backblaze. There are many options to choose from. On a Mac Arq7 is a solid choice. It will also take care of dataless files, which are missing from Time Machine but still need to be backed up.