BLOCK files are encrypted versions of BLEND files (for the Blender 3D graphic program). They are supported by the third-party add-on BPPlayer. This does not seem to be actively developed or supported now; online references to the program or format tend to go to forum threads asking about it which in turn link to other forum threads, the most informative of which has succumbed to linkrot (though the Wayback Machine has it... that's an invaluable resource). There's still a download page for a version of it.
Sections of BLEND files are referred to in the Blender documentation as "data blocks", but this is entirely unrelated to this sort of BLOCK.