According to Jim Starkey, the person who coined the term, "Blob don't stand for nothin'." However, it is often referred to as a "Binary Large OBject", meaning a large file with content not easily readable by people.
With an open source project, you have source code which is turned into executables/"blobs" by the compiler. As long as you trust the compiler, you can (functionally) know the content of the blobs by looking at the source code they were made from.
In the case of Ventoy, several "blobs" are included from an unknown or vague origin. This is a great way to bundle malware, as seen with the XZ backdoor from earlier this year. As such, the original creator of the linked issue is requesting they are built/obtained at compile time, so either the content or origin of these files can easily be found.
SteamOS is not the same as its base Arch Linux. If you want something slightly easier but still Arch-based, try EndeavourOS (but please not Manjaro).
If you have the time, try switching on your own terms within the next year. It's almost guaranteed you'll run into issues, but trying to dual-boot now rather than later gives you all the time you need to figure it out before MS forces you on Windows 11.