Tranzschelia pruni-spinosae affects the primary host Prunus spinosa. Brown uredinia and black telia are carried on the leaf undersides.  Aecia and spermogonia are on the alternate host, Anemone sp.   It is not a gall-causer on either host. 

Tranzschelia discolor

microscopy

Photograph the smut and the host; note the host-plant in your comments.  We recommend that this is confirmed by an expert before submission to Naturespot; microscopic examination may be needed

Prunus speces, including blackthorn