blocks=Partition[Characters[ToLowerCase["BOXKDQCPNAGTRETGQDFSJWHUVIANOBERFSLYPCZM"]],2];
ClearAll[DoStep,ABCBlockQ]
DoStep[chars_List,blcks_List,chosen_List]:=Module[{opts},
 If[chars=!={},
  opts=Select[blcks,MemberQ[#,First[chars]]&];
  {Rest[chars],DeleteCases[blcks,#,1,1],Append[chosen,#]}&/@opts
 ,
  {{chars,blcks,chosen}}
 ]
]
DoStep[opts_List]:=Flatten[DoStep@@@opts,1]
ABCBlockQ[str_String]:=(FixedPoint[DoStep,{{Characters[ToLowerCase[str]],blocks,{}}}]=!={})