Distribution of blue books causes depression (6)
I don't think this will translate across the pond, but in US universities "blue books" are sometimes used to administer exams; professors pass out the booklets (colored blue for some reason) for students to write their answers. And what could be more depressing than having to take an exam? And, Chambers thesaurus equates depression with "a bubble in glass." Hm, when the explanation is 10x longer than the clue....