Converts starch into maltose. In contrary to other maltogenic alpha-amylases BlmA cannot hydrolyze 1,4-alpha-glucosidic linkage next to 1,6-alpha-glucosidic linkages. hydrolysis of (1->4)-alpha-D-glucosidic linkages in polysaccharides so as to remove successive alpha-maltose residues from the non-reducing ends of the chains. Belongs to the glycosyl hydrolase 13 family.