Hello mmarkovich and welcome to the forums!
A blacklist is used to explicitly exclude updates from being downloaded or installed.
The "not found" message indicates that an update is missing in your repository. Have you run the Update Generator with the appropriate settings for your environment (correct OS)?
In order to give you more (precise) information, we would need some more background. For example, an excerpt of the installation log (C:\Windows\wsusofflineupdate.log).
Greetings
Gerby