Urocystis ranunculi

Urocystis ranunculi is a smut fungus which affects buttercups.  Yellowish swellings or pustules occur on leaves, petioles, stems, etc., eventually erupting to release a mass of dark spore-balls.   Even when immature the dark mass can be seen inside the unopened blisters when held up to the light.  The spore-balls have a central brown spore surrounded by a few hyaline sterile cells.

Entyloma microsporum causes yellowish pustules on buttercup leaves, but does not have black sooty spore-balls. 

host

Photograph the smut and the host, ensure your photos show the dark sooty spore mass.  Note the host-plant in your comments.  

Buttercups