My reason for "Dark Pact" for the Blackstaff book is that I think that whomever takes up the Blackstaff makes a pact with someone, or something. Also there could be a reason Harry has to take up the staff similarly to taking up the Winter Knight mantle.
"Dark Pact" is fine, it was the "Black Staff" one that seemed too pointed.
I don't think Jim, or the publisher would use a book title with profanity.
Jokes, man, just jokes. I just liked the double meaning, and the image of harry saying it as REALITY ITSELF IS RENT ASUNDER AND HORDES OF NIGHTMARISH TERROR POUR THROUGH. Because, what else is there to say?