Catalyzes both the conversion of demethylsterigmatocystin (DMST) to sterigmatocystin and the conversion of dihydrodemethylsterigmatocystin to dihydrosterigmatocystin (DHDMST) during aflatoxin biosynthesis. 6-demethylsterigmatocystin + S-adenosyl-L-methionine = H(+) + S-adenosyl-L-homocysteine + sterigmatocystin Mycotoxin biosynthesis; aflatoxin biosynthesis. Belongs to the class I-like SAM-binding methyltransferase superfamily. Cation-independent O-methyltransferase family. COMT subfamily.