I've had EXACTLY the same thing in mine for several years now. I've done no testing other than to determine the black residue is not oily. I pulled the coolant tank off to try to get the residue out, but it was stuck good, and I was not able to eliminate it with either solvents or Tide, yet it doesn't seem to accumulate. I've pretty much concluded there is some kind of minor seepage of combustion gas & residue leaking into the coolant either from a small leak in the head gasket or a crack in a combustion chamber. The engine runs fine, has power (such as can be provided by 72 HP!) with no over heating, no loss of coolant, no increased pressure in the cooling system nor any other negative effect. Even in hot weather, 100°+ with AC running, the temp never exceeds 100C and usually runs closer to the 80C mark. There's no evidence of over pressure in the coolant such as you'd expect from a significant combustion leak.

Recently I developed a leak in the coolant tank and replaced it with a new one. Withing days I could see the tank developing the same residue.

I don't like it because it doesn't seem normal, but with nothing negative about the performance I've had little motivation to sort it out and eliminate the 'problem'. I'll be very curious about what you determine yours to be as I'll bet mine is exactly the same thing!