Black soot on the jar rim and a grey mark on the wall above my candles

A flame that grows through the burn is a fuel supply problem, not a trim problem. CD-10 in an 8oz straight sided jar is a size or two up from where most people land with that wax and that much oil. Drop to CD-8, burn it four hours, and look at the flame height at the two hour mark rather than the melt pool.

I burned this exact comparison last winter because I was sick of guessing. Four identical 8oz jars, same 464, same 8 percent of the same oil, wicks CD-6 through CD-12, four hour burns, photographed the flame at 30, 60, 120 and 180 minutes and weighed each jar before and after.

The CD-12 was throwing visible black by 80 minutes and burned through noticeably more wax per hour than the CD-8. The CD-10 was the interesting one: clean on the first burn and sooty by the third, once the pool got deeper and the glass was holding heat. That is very likely what you are seeing on a shelf jar you have burned a few times. The CD-8 never sooted but left a thin ring on the glass for the first hour, which self corrected on the second burn. I sell the CD-8 version now and nobody has ever complained about the ring.

I did this to myself in the other direction and it cost me a market. First season I had tunnelling, so I sized up. Still tunnelled slightly, so I sized up again. By December every jar I sold was sooting the glass black within two burns and four people messaged me about it. I remade about forty jars at my own cost, and the fix was going back down two wick sizes and accepting a slower first melt pool. Nobody has ever returned a candle for being slow. They absolutely return them for black glass.

I run CD and ECO side by side in the same 8oz jar because I could not get a straight answer either. ECO tends to curl the tip into the pool and self trim, which keeps the flame shorter for longer, and in my jars it sooted noticeably less at the same nominal size. The tradeoff is that ECO is fussier about fragrance and I get more mushrooming with vanilla heavy oils. If you are already at the edge on load, two test jars with ECO are worth doing before you buy a whole new box of CDs.

Going to gently disagree with the wick-only answer. You are at 8 percent in 464, which is at the top of what that wax holds without weeping oil into the pool, and free oil at the wick is a very efficient soot machine. Before you rewick everything, run one jar at 6 percent with your current CD-10 and see whether the black goes away. If it does, you have a load problem wearing a wick problem's clothes, and sizing down would have just handed you tunnelling instead.

Long shot, but where is that shelf? A candle under a shelf, near a doorway, or in the path of an air vent flickers constantly, and a flickering flame soots even at the correct wick size. I chased this for a month before noticing the jar sat two feet from a ceiling vent that kicked in on a thermostat. Moved it three feet, same jar, no soot.