According to the stat() documentation, blksize is the "blocksize of filesystem IO". This corresponds to the GridFS chunk size.

The blocks field is always the "number of 512-byte blocks allocated" and has no direct relationship with blksize (see: this documentation). While we could easily calculate this value, it's not relevant to GridFS and users might be confused in thinking that it is actually the number of GridFS chunks. We should continue to leave it unset (i.e. 0).