Hey there occulties,
Since everyone thinks the slightest tinge of black is a thing, the following is probably an 8 or so on a scale of 1-10 insofar as black burns. You'll notice the end has been blown off where it starts burning clearer. That's because it worked well, but the target put up one hell of a fight (but lost, mwahahahaha.)
So, if you get a bit of black around the mouth of the chimney or a tiny black spot, that's not exactly what I'd term a black burn on a candle.
Here below, I have something that might rate a 3 (or maybe a 4 at best if I'm being hugely generous) on the "burning black 1-10 category" meaning it's pretty mild, and could be much worse.
Notice I scraped a little black off on the mouth of that to give you an idea of how black it is...there is a clean spot, and that's post-ritual. ;)
Now in that above picture, I'd not even have been too concerned over the quality of the burn, excluding every candle burned on that case burned with that much singe. Remember, do NOT look for signs, or you will only project them where there are none. More often than not, the problem is caused by physics, and not a sign. If you get a burn like the above second picture, just burn another seven day candle on the same problem and to the same end. Singe-y burns are often caused by too much herb or oil on the seven-day-vigil, and shouldn't cause a slobbering panic...in fact, even if it is a legitimate black burn (meaning some obstacle is impeding the influence...if it burns clear at the bottom, you're probably OK,) you shouldn't ever have a slobbering panic. Do some road opening, keep working. You'll get through.
Hope that's helpful, everyone. :)