Terrible tirade overwhelms me, not wanting a black mark (7)
I believe the answer is:
demerit
'black mark' is the definition.
(a black mark marks a demerit or fault)
I don't understand how the rest of the clue works.
(Other definitions for demerit that I've seen before include "Mark given for a fault" , "Something deserving censure" , "Conduct that deserves censure" , "Feature deserving censure" , "Fact deserving censure" .)