That is carbon. Your copper washer wasn’t sealing completely and the combustion was leaking into the injector bore. You have to clean it out. Many say a 12 gauge gun brush is the right size.
I borrowed this from another post.
Under no circumstances let carbon fall into the cylinder UNLESS it is at TDC. At TDC there is virtually no space for carbon to accumulate in the cylinder and it is held in the precup. With the injector and the GP removed you can vacuum out the carbon and use a .....
brush
The timing mark is found on a little plate that is on the front of the engine on the passenger side. The plate is mounted so it is very close to the main pulley on the lower front of the engine which is mounted on the vibration damper. On the side of the vibration damper, is a line, which when lined up with the line marked (O) on the plate, tells you that the #1 piston is at TDC(top dead center) You can see the line on the damper, and it points to the line on the plate with the (barely visible)O mark on it.You'lle to find a way to turn your engine over by hand. Make sure you remove the power wire to the injection pump(so the engine won't start)and then rotate the engine until you can line up the marks.
The engine rotates in a clockwise direction as seen from the front of the truck, and for every 1/4 turn of the engine(crank) each next piston will arrive at TDC. So, When the line on the vibration damper is at the 2 oclock position, cylinders 2 and 5 will be at TDC. When the mark is at the 5 oclock position, cylinders 7 and 6 will be at TDC, 8 oclock has cylinders 3 and 8 at TDC, and back to 11oclock, 1 and 4 will be at TDC.
Cylinder numbers are assigned 1,3,5,7 on the left(passenger) bank from front of truck to back, and 2,4,6,8 on the driver's side, front to back. Firing order is 1,2,7,3,4,5,6,8.
