This is sort of a confused article. blob: URLs (not blob in the url) are references to local data. They can't really be downloaded (well if they are still live you probably can) and typically only contain a small slice of the video (timewise).
What this is doing is finding the playlist file that describes this video and playing this (or using a player like VLC to stitch these chunks together).
Then it has other instructions to use different tools for specific sites.
So really the only reason blob: is relevant at all is because it is what you might see if you try the "simple" solution of right-click save. The actual article doesn't deal with the blob: URL at all.
So guess the more accurate title is "Are you trying to download a video but it has a blob: URL? There is a decent chance that this is using DASH or HLS under the hood. You can try to find the playlist like this: $find_m3u8 and then stitch that into a single video like $vlc".