Catalyzes the methylation of C-1 in cobalt-precorrin-5B to form cobalt-precorrin-6A. Co-precorrin-5B + S-adenosyl-L-methionine = Co-precorrin-6A + S-adenosyl-L-homocysteine Cofactor biosynthesis; adenosylcobalamin biosynthesis; cob(II)yrinate a,c-diamide from sirohydrochlorin (anaerobic route): step 6/10. Belongs to the CbiD family.