He cursed and opened his eyes
Why not have him actually utter some kind of curse? You clearly have no problem with using strong language, which is usually why people say someone cursed without saying what the curse is. Plus, this would provide some flavor-- what do they curse by? What do they consider profane?
Taken with his lightly tanned skin and blue eyes, the man was clearly a black humorist.
I take it that this means his dominant humor is black bile? If so, why not use the classic name and say that he was "clearly melancholic"? I'm familiar with the medieval idea of humors, but I still thought of Bill Cosby when I read "black humorist."
He couldn’t decide whether she was of the black or blue humours.
Okay, since you have more humors than the classic four, and since you spell it
humour here, why not say that the man is a black
humourist. Also, why do you italicize it here, and not above? I don't care which, but I think it ought to be consistent.
“Other’s might say we poisoned you,” the woman added.
others
if all you desired was to talk you had but only ask. Especially you, my lady,” he added to the woman. ... He often thought such things while in the presence of attractive women, but never before had he the balls to speak them.
Such things as: she only had to ask him if she wanted to speak? This is pretty tame. This is my introduction to the character, but he must be a shy fellow indeed if this is the kind of thing he never had the guts to say before. Or are you talking about the f-bomb he dropped? That doesn't make sense because it talks about how he often thought such things, which really seems to be about the flirtiness, not the dirtiness.
“We are of the Sicarii,” the woman said.
The Sicarii are totally cool. Does this assassins' guild take its name from the historical Sicarii? Your story seems to take place in Renaissance Italy, so I think that's very plausible. If not, are you sure you want to borrow the name of a real organization like this? Just something to think about.
Even Cold Ones bleed, Mathieu.”
Cool. What are Cold Ones? Have they been introduced already?
“We are…an equal opportunity employer,” the man said dryly.
This is a great dry joke for anyone who has lived in America since 1964. For anyone else, I would change this. Maybe: "We are not what you would call... discriminating." Or: "We are not so.... noble as that."
He would take even a half chance over no chance any day. “Non.” He shook his head emphatically. “Non, I am sorry but I cannot.”
Okay, he has seen the face of the secret master of the secret guild of murdering poison assassins. How on earth can he think that refusing this man is half a chance? How can he think that this refusal will earn him anything other than death? This guy is supposed to be a realist...?
Okay, I have to stop here because I have class, but I'll finish reading and reviewing later. So far, I like it a lot! Very fun!