Is a person sinful if he says to his friend: "You are blindly short-tempered, and I am accountable before Allah for every word I have said"?

"Blind fanaticism" is not a recognized term, but it refers to excessive fanaticism for something, even if it is false. Reprehensible fanaticism is fanaticism for falsehood, and it is an act of the جاهلية (pre-Islamic ignorance), as Ibn Taymiyyah mentioned. It is permissible to say this to a person if he is indeed fanatical, but it is not permissible to accuse him of it if he is innocent of it.