Rengar gains Ferocity stacks when one of his spells is blocked by a spellshield.

When a champion such as Nocturne, Sivir, or Malzahar blocks the damage of one of Rengar's abilities (Bola, Roar) with a spellshield, Rengard still gains a Ferocity stack. 100% reproduction rate with Bola and Roar. Rengar's passive says "Rengar generates 1 Ferocity whenever he damages an enemy with an ability." Since he is not damaging enemies when they spellshield an ability, he should not gain Ferocity.