How to disable malware warnings for specific files in macOS?

What is the warning message, exactly? Does it actually mention "malware", or does it say "is a file downloaded from the internet... are you sure" or something to that effect?

I have not done this to many files at once, but if, as I surmise, this is a matter of the files having the quarantine flag set, you may be able to remove them all at once using a simple command in Terminal.

1) Open Terminal

2) Type "xattr -d com.apple.quarantine "

(without quotes but WITH a trailing space, as above)

3) Drag the files onto the Terminal window

4) press enter

In addition to the precious questions about the error message, how exactly did these files get retrieved from Backblaze, and where were they placed?

This may well be quarantine, which would be typical for downloaded files and can be removed. The xattr command can process multiple files, and can recurse through a directory and its subdirectories too. (details)

If you have the whole project under one directory and its subdirectories:

xattr -r -d com.apple.quarantine /path/to/directory

It is difficult to cause wider trouble with this particular command, as it solely removes the quarantine attribute. Nada más.

Or this might be some other issue.

Yes, What is the warning message, exactly? Is it something like "image.webp Not Opened. Apple could not verify image.webp is free of malware that may harm your Mac or compromise your privacy". I saw that in recent macOS versions but new versions have fixed this for me (another cause could be insufficient permissions which could also be fixed for all such files with one command).

If yes, use either Terminal as Luis suggested (so you don't have to visit Privacy & Security > Security to be able to open each and every file). GraphicConverter 12.5 also has an option to reset com.apple.quarantine flag (unlike Terminal command it does not completely remove the flag):

https://discussions.apple.com/thread/256029589?answerId=261336737022&sortBy=oldest_first#261336737022