alma mater vs blackboard

alma mater

noun
  • A school's anthem or song. 

  • A school, college or university from which an individual has graduated or which he or she has attended. 

blackboard

verb
  • To use a blackboard to assist in an informal discussion. 

noun
  • A large flat surface, finished with black slate or a similar material, that can be written upon with chalk and subsequently erased; a chalkboard. 

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