From what I can see, it's more like a specialized ZIP-like compressor for Vorbis/Opus files. You archive them on blr, and decompress them back to the original format.
Kinda like what Dropbox Lepton did for JPEG back then.
Simpler: it's basically ZIP but specific to one format.
I just tested this out for myself and I was unable to play the .blr file in VLC or using ffplay. It is not recognized as anything by ffprobe either. So it seems like you need to decompress it to play it in a standard player unfortunately.
There is a long history of compressors that replace the entropy coding layer of a codec with something else entirely. This is exactly it.