What is the clockworkmod blobs folder good for?
Starting with version 6, deduplication support has been built into clockworkmod recovery. The blobs directory folder contains a hashed directory structure that holds the deduplicated files across all backups.

See the developer's Google+ post for more, in short:


  TL;DR:
  ClockworkMod Recovery now deduplicates files between builds. This results in way smaller backups.
  Don't delete /sdcard/clockworkmod/blobs