Abstract: Blue-black color favored in medical imaging is imparted to black and white photographic emulsions by the addition of a compound of formula I or II: ##STR1## wherein: R.sup.1 and R.sup.2 independently represent one or more carbon atoms necessary to complete a 5, 6 or 7 membered ring;R.sup.3 and R.sup.4 independently represent H, alkyl or aryl groups or together represent the atoms selected from C, N, O and S necessary to complete a 5, 6 or 7 membered cyclic ring but are not both H;R.sup.5 and R.sup.6 independently represent H or alkyl or together represent the atoms selected from C, N, O and S necessary to complete a 5, 6 or 7 membered ring;X represents a bond or a divalent linking group,each Y may be the same or different and is selected from S, Se, O, and NR.sup.7, where R.sup.7 is H or alkyl of up to 5 carbon atoms, andZ is S or Se.