Cast Iron vents expelled black substance overnight

Unless there was a fire somewhere nearby and the smoke got pulled in through the rim joist area that didn't happen overnight.

If the paint is new it could have bled trough that over a few days.

It's normal for any duct and dirt to get in the air current and get on the walls above any type of heater. But that may be more than dust or dirt.

This reminds me of a service call I received from a new customer.  Single mom with a teenage daughter.  They had electric heat.  The baseboard type, so there was no boiler or furnace in the home.  This customer insisted that the electric was making soot. and the electric baseboards had those black shadow markins you photos show.

This was a stumper for me… so I looked at each electrical connection box to see if there was any burnt insulation in any of the baseboards.  Nothing.

Now I’m an oil burner guy, and I deal with SOOT on a regular basis.  Maybe that is how I got recommended.  But I looked at all the rooms in the home, except for the teenage daughter's room.   I asked the mother if the daughter's room also showed the  same markings.  She replied yes.  I asked if I could look in the daughters room, (it was locked).  And mom replied that she would prefer that she would prefer the daughter be home for that.  

After school I returned to find the daughter home and the room looked like this.  

Well maybe I'm exaggerating a little.  But there were lots of cheap candles in the room and they were all lit.  Cheap wax is a great way to get incomplete combustion from that fuel.  The Wax is the fuel.  Actually the Paraffin in the wax is the fuel and the lower cost candles have a lower percentage so the other components of the fuel "no burn so good" and make soot that floats thru the air.

As the carbon particles move through the heating elements the air flow is accelerated taking the carbon with it.  As the air crashes against the wall the air bounces out of the way because the wall changes the air direction rapidly. The carbon particle has a slightly larger mass than the air molecule so many of the carbon particles don't change direction as fast. (it’s that old Newton’s Laws of Physics thing where “an object in motion…” yadda yadda yadda)  so the carbon crashes into the wall and sticks there.  Eventually enough of those carbon particles stick to the wall that it changes the color.  And if there is enough carbon in the air on any particular day, that can happen pretty fast.  

For 2 visits to the home, all I got was the loss leader low cost "Diagnostic Fee"

Win some Lose some

Actually the combustion process is the same as with any other fossil fuel. Incomplete combustion will have a higher smoke spot number that a burner with complete combustion. the only adjustment you can make on a candle wick is the pool of liquid that is wicking up to the flame then evaporating into a vapor so it can burn. The combustion air is comming from the air around the flame. There is no adjustment for combustion air in that type of burner. (the candle being the burner). the only adjustment is the quality of the fuel. If you have cheap wax then you have less fuel in the total container. (the container being the candle) the other stuff in the container will literally "go up in smoke" because of incomplete combustion.

As far as the light being a result of soot, I beg to differ. I have noticed that oil that burns sooty has a more orange-yellow color, while flames that burn with a zero smoke spot test, burn bright white. (that is oil burners i'm referring to). And since a candle must first melt into a liquid to wick up to the flame and then needs to vaporize into a gaseous state to actually burn, I think that the combustion process is very similar, and the less soot in a candle flame the brighter the light from that flame.

But I could be mistaken, Probably not, but there is a first time for everything.

I will add another observation. I built a living room addition on my house 27 years ago. It has a fireplace and my wife burns scented candles and a Dynatherm boiler in the basement running with a trace of smoke in accordance to the manual. After 20 years or so we could see ghosting of the wall studs on our tall gable wall. Soot or dust obviously clung to the coldest drywall touching wood, a thermal bridge, more than the insulation backed drywall. It came off with spic and span and elbow grease.

Years back when I started installing cast baseboard I was to to put foil behind the baseboard and tto fold it over the top to make a seal. I was told that was to prevent streaking on the wall behind the baseboard. I'm thinking that is what you have here!

My theory is that this is ambient dust attracted to a film of condensation that develops on the inside surface of poorly insulated outside walls when the heating convectors are cold. When the convectors are hot, the airflow delivers ambient dust that sticks to the wall until the condensation evaporates.

This condition may be related to elevated indoor humidity. I would run the boiler and check for steam leaking at air vents, shutoff valves, and pipe fittings.