The dark fuel could be dark because it's flushing out old after run oil, old dried up castor oil, or it's flushing out surface rust from the bearings. If the engine seems to turn over relatively smoothly, the bearings aren't so far gone that the engine shouldn't run, however. Adding a bit of fuel to loosen things up was the right thing to do.

If you have a bunch of gunk coming out of the engine, it could have fouled your glow plug. I'd try a fresh glow plug (not an expensive one, like a Tower Hobbies or Super Tigre plug) and I'd open up my low speed and high speed needles. If the needles are too rich, the engine will run poorly; if they're too lean, however, then it won't run at all.

Get a clean, new piece of fuel tubing and connect it to your low and high speed needles and blow through the tube. This will allow you to make sure that the needles are open enough and that nothing is blocking or fouling them.

You simply need a hot glow plug, fuel, and compression to get the engine started. It sounds like you have compression so simply check the above items to assure heat and fuel. Good luck.