block out vs let up

block out

verb
  • to prevent from entering or penetrating. 

  • to cover something, so as to make it impossible to see. 

  • to prevent (a thought) from entering one's mind. 

  • to begin to reduce to shape; to mark out roughly; to lay out. 

let up

verb
  • To cease or stop (doing something intense). 

  • To cease or stop. 

How often have the words block out and let up occurred in a corpus of books? (source: Google Ngram Viewer )