Is it necessary to have a TimeMachine backup of the Backblaze Directory? Since I noticed TimeMachine continuously backing up large files, I had a look with tmutil compare to see where those files are located. Since I had not worked on my mac book air but used it to listen to music I suspected the iTunes library. 

As it turns out TimeMachine is continuously backing up /Library/Backblaze.bzpkg/.

Do I really need a backup of this directory? I see the point that in case I have a hardware failure but my TimeMachine is still working I might need to re-upload everything to Backblaze again?

What would be arguments against putting /Library/Backblaze.bzpkg/ into the excluded items list of Time Machine?
This file is where Backblaze stores information about what is stored on their servers and copies of files being uploaded or about to be uploaded. I would exclude this from Time Machine backups, as it will often change and contains nothing you're not already backing up elsewhere.