The smut fungus Urocystis violae causes pustules on the petioles and leaf veins, and/or leaves of violets.  The pustules rupture to release a black spore mass, composed of many individual balls of spores.

host

Photograph the smut and the host; note the host-plant in your comments.  Ensure that your photos show the dark spore-mass.

Violets - including Viola odorata, V. riviniana, V. reichenbachiana and V. hirta, and garden cultivars 

Spring

Rare; the 2023 record is the first known for VC55.