blackthorn vs buckthorn

blackthorn

noun
  • A large shrub or small tree, Prunus spinosa, that is native to Europe, western Asia, and north Africa. It has a dark bark and bears thorns. 

  • A stick or staff taken from this tree. 

buckthorn

noun
  • especially genus Rhamnus, 

  • also genus Frangula, especially Frangula alnus, the alder buckthorn. 

  • especially the common buckthorn or purging buckthorn; Rhamnus cathartica. 

  • Hippophae (family Elaeagnaceae), the sea buckthorn. 

How often have the words blackthorn and buckthorn occurred in a corpus of books? (source: Google Ngram Viewer )