Mageblood in PoE 2 uses a Utility Belt base, not a special hidden belt base. The item itself is the unique belt, and its base type is listed as Utility Belt at level 55.

What that means

  • If you mean the base item type to look for: Utility Belt.
  • If you mean how to get Mageblood specifically: it can appear as a Ritual reward, drop from bosses/rare enemies, or come from other very RNG-heavy methods.

Quick note

There’s also older Path of Exile forum discussion about Mageblood/Headhunter sharing certain chance-orb base considerations in PoE 1, but for PoE 2 the relevant listed base on the unique is still Utility Belt.

TL;DR: the base belt is Utility Belt.