RE: Play in engine's crankshaft?
The play is called end play. Put a prop on it and tighten it down then check it. If there is a problem it will show up on the back plate. Just remove the four screws holding that on and look at it. There will usually be some slight swrils on the back plate. If it has deep grooved swirls then you have a problem.
The dark gray mater in any new engine is just the metal of the engine getting worn in, they all do it. If it's an old engine then there will be more and more, darker and darker as the engine is wearing out. Loose end play and the gray mater blowing out the pipe tends to freak out people that have just bought new engines. If the engine is real tight there will be more of it. Just normal. This bit of metal will often tend to help burnout glow plaugs quicker, just normal. Rebuild an old engine and it will happen again, normal. Have a loose muffler or muffler baffle and you will see a big bunch of gray mater.