What is HH blood type?

Perhaps you mean the hh blood type. The hh blood type is the rarest blood type which does not express the H antigen present on virtually all RBCs as the building block for other antigens.

The H antigen (genetically HH or Hh) is a surface protein on RBCs that is found in All the major blood groups. The H antigen is the building block for the other surface antigens A and B.

Th hh blood type (also called the Bombay Blood Group because it was first discovered there in 1952), is that in which this protein cannot be expressed.

A person with this blood group would have to receive blood only from another with the same hh blood type, as the blood would react with every other blood group.