Blackjack never liked Weasel Firefox but he kept him around as a dragonthrall (i.e. in his service in return for longevity) since he was a useful source of information. Up until the point Weasel struck tried to double-cross him. Black' wouldn't have taken it quite so personally if Shade and Soul hadn't been dragged into the matter and the three Daemonslayers almost killed.
He bides his time and eventually takes revenge on Weasel: cutting his throat and leaving him for dead. What he doesn't realise is after he leaves, Saragoth turns up and makes Weasel an offer he can't refuse... I'll probably put a better explanation on my site once I get Weasel's bio done.