Didymos - yeah the barbed wire spell was black magic. There have been several other instances Harry has encountered truly dark magic, mostly when involved with Outsiders, the Black Council and Black Court Vampires.
Mira - I think what he said was that Cowl's magic wasn't fully dark (like Corpsetaker's for instance), but had elements of dark in it, just like Harry's own.
Half vamps were hit in their vamp half, though, not their human half. Even if their human half had lost the ability to live without the vampire.
And yes, Harry killed (not murdered) Susan. He did so with a knife, though, not with magic. Harry has straight-up-murdered RCV's with magic, though, and that never seemed like black magic to anyone (or he would've been kicked to the curb for murdering Bianca with magic to kick off the war).
So, to sum up, I think Harry used an evil ritual, but it was not black magic, and he used it for a good purpose.
So if you hit a person with your car, and they get injured but in the end infection kills them in the hospital - are you less responsible than if they had died right there on the road? Harry may have only intended to kill Vampires, but by killing the vampire half of the St Giles half-vamps, he still ended up killing mortals.
I think you missed the point of Jim's quote...the consequences are cosmic in nature. Just because the White Council might not think something is a violation, doesn't meant he universe doesn't. Killing someone has consequences, both mortal and supernatural, whether magic is used or not. And should you use magic (including ritual) the consequences upon you will come from the universe - the black magic corruption is one consequence, and I suspect that getting your just desserts (the whole what goes around comes around thing that Michael talks about) is another.
The quote I gave is in response to a question about how it doesn't seem to matter how many Fae you kill with magic. Jim's answer first says "It hardly seems fair, doesn't it?" and then the rest of the quote about how magic morality works. So Harry killing Bianca with magic has consequences, just not necessarily all from the White Council - although I would remind you they were prepared to throw him to the wolves several times over it.
Also, there is another quote by Jim about how intentions matter but the ends matter more - especially when it comes to magic. You might of course disagree with that, but that's how his book universe works.
But if the substance of the consequences of the act itself does not have its own inherent quality of good or evil, then how can the /intentions/ behind it determine a similar quality? “Really, I was only trying to provide a better quality of life for my family and my employees. It wasn’t my intention to destroy that particular species of flower in the rain forest that cures cancer.” “I was just trying to give those Injuns some blankets. It wasn’t my intention to expose them to smallpox and wipe out hundreds of thousands of innocent people.” “I just wanted to get that book finished while working two jobs and finishing a brutal semester of grad school. It wasn’t my intention to screw up the name of Bianca’s personal assistant whose death had motivated her to go all power hungry to get revenge on Harry.”
There’s some old chestnut about good itentions serving as base level gradiant on an expressway that goes somewhere, but I can’t remember the specifics right now. 
While I agree that the /intentions/ of the person taking action are not without significance, they carry far less weight than the /consequences/ of that action.
“I meant to shoot him in the leg and wound him, not hit the femoral artery and kill him, so I should not be considered guilty of murder,” is not something that stands up in a court of law /or/ in any serious moral or ethical evaluation. You had the weapon. You knew it was potentially lethal, even if you did attempt to use it in a less than fully lethal fashion. (Or if you DIDN’T know that, you were a freaking idiot playing with people’s lives, something really no less excuseable.) But you chose to employ the weapon anyway. The consequences of those actions are /yours/, your doing, regardless of how innocent your intentions may have been.
Similarly, if you meant to drill that ^@#%er through the eyes, if you had every intention of murdering him outright, but you shot him in the hand and he survived with minor injuries, again the consequences overshadow your intentions. You might have made a stupid or morally queestionable choice, but it isn’t like anyone *died* or anything. He’s fine (at least in the long term), you’re fine, and there are fewer repercussions–regardless of your hideous intentions.
The exercise of power and the necessity to consider the fallout from your actions isn’t something limited to wizards and gods. Fictional people like Harry and Molly just provide more colorful examples.
As for violating the laws of magic themselves turning you good or evil, well. There’s something to be said on either side of the argument, in the strictest sense, though one side of the argument is definitely less incorrect than the other. But it’s going to take me several more books to lay it out, so there’s no sense in ruining the fun.
Jim
Bad Alias is correct in that human sacrifice as part of a magic ritual counts as black magic, both to the White Council and the universe.
Dina - the ultimate goal of the ritual is to
kill. It doesn't care who. That's still evil. The quote I previously showed to Avernite starts with this:
"Note also the killing law only applies to Humans.
You can kill as many faeries as you want with magic."
Bingo. It hardly seems fair, does it?
The Laws of Magic don’t necessarily match up to the actual universal guidelines to how the universal power known as “magic” behaves.
I have bolded the question that was asked to Jim that preceded his answer about how magic lines up. Just because you don't make a gun doesn't mean when you shoot and kill someone, you're any less guilty.
