The blood sample has only antigen B on the red blood cells and antibody A in the plasma. This indicates that the blood group type is B.
The negative and the positive sign of the blood group is determined by looking at the presence of another antigen called the Rhesus factor or RhD factor. The presence of RhD antigen in RBC is denoted by a '+' sign next to the blood group.
Since the question does not mention about the presence of RhD factor, the blood can be either B+ or B-.