sle2115
NRA LIFE MEMBER
In a pristine, new and clean, fuel-system, such as you obviously have, that black could very well be from engine oil that has gotten in the fuel system, either via the mechanical lift-pump, or possibly from the injection-pump, although it is usually the other way around, fuel leaking into the crank-case through either of those points of entry.
A fuel-pressure gauge, installed between the filter and injection-pump, is a wonderful trouble-shooting aid, and it will warn of a filter blockage before it just quits running from lack of fuel.
There is no mechanical fuel pump! It's in the dump and bypassed, as well as having a block off plate. Could have just been all that clean fuel cleaning out the system, or nasty fuel, I don't know, didn't look like dirt, just a dark gray color.