Alternaria brassicicola

From ISTA's Common Laboratory Seed Health Testing Methods for Detecting Fungi:

Black, shiny growth consisting of conidia in long, narrow chains, is characteristic of the species. Chains simple, rarely branched, beaks not visible. 

Conidia straight, cylindrical usually tapering slightly towards the apex, basal cell rounded, mostly with transverse septa (1-11), longitudinal septa rare, pale to dark olivaceous brown, smooth or becoming slightly warted with age, 18-130 x 8-20μm.

Since both A. brassicicola and A. alternata have long chains and they appear commonly together on seeds of Brassica spp., their identification becomes difficult. A. brassicicola is more shiny, black and condensed in its growth than A. alternata, which is more fluffy, lighter in colour and with rather visible beaks.