It is a polishing tool for the hi ( grooves) in a sword.
This groove is called a fuller in English. A bo-hi, is a wide round bottomed groove. ( This should not be confused with - Boshi, which is the pattern of the sword tip; Bohi, which is a tombstone; or Boin, which are big knockers. Messing with the wrong boin, can get you acquainted with the boshi and bo-hi, which might get you a bohi.).
The groove is cut with a scraper called a sen ( actually a specialized sen).Once carved in the sword, it is sanded with a tool like Erik's. Then it is burnished with steel tools called migaki -bo and migaki -bera.
Stacy
BTW, Blood Groove is a made up term.