Black to me, does not usually mean bad fuel, would instead look for decaying rubber, with fuel lines from tank to filter ALL ok, may indicate a fuel pump diaphragm going out, this can also lead to a "leak down" of fuel going to the engine. At first a few turns of the motor builds up regular pressure.
did you have to force or squeeze one of the rubber lines to go onto a steel hose, or fitting when they were all changed?
Are there any old rubber lines left between tank, and filter?
Did anyone else put something other than gas into the tank?
rust would be brown collared in filter.
Keep us in the loop, good luck.