Black burner exhaust smoke usually indicates:
a. Too much fuel: Fuel pressure too high.
b. Not enough air: Improper air fuel mix, possibly caused by low voltage or bad blower motor.
c. Low voltage will cause slow rpm of blower motor resulting in improper air / fuel mixture and poor combustion.
From what you have described, I suspect you have a blower motor or fuel pump problem.
Check and see if the fuel pump shaft turns freely, if not this could cause drag on the blower motor, which would
in turn cause the blower motor & fan to turn slowly this would result in black exhaust smoke. If the burner motor
is slow too start and the fuel pump turns freely, I would suspect burner motor.
If you like call me it is 5:23 AM here in California, I will be at the shop by 7:00 AM.[whatup]