The smut fungus Ustilago avenae infects the spikelets of various grasses, including Oats (Avena) and False Oat-grass (Arrhenatherum elatius).   The spores disperse readily and appear black in the mass, but when viewed at high magnification are seen to be lighter on one side.  They are small and spherical, and spinulose.

host

Photograph the smut on the host grass.  In your comments, note the species of grass affected.  We recommend microscopic examination of the spores.

On various grasses, including Oats and False Oat-grass