I flushed my car's radiator a couple of months back, was running plain water before...
Luckily all went well, no much came out of the radiator or the engine block (kachra and gunk i mean)..
But when i checked the reservoir it was full of this black fungus like thing... I had no idea what it was but drained it completely ofcourse...
Running caltex poison now...
You should simply clean the reservoir bottle only if the coolant in the radiator is fine...
Check after a few days if the blackness returns... Flush complete system then...
Cheers...