Hi rfriesen -
If you are running el cheapo 2-cycle oil for AIR cooled engines (not the more expensive synthetic oil), I recommend you increase the oil concentration to around 24:1 if your engine is still in the break-in phase. It is normal for that black oily goo to drip out of the exhaust pipe. The oil does not burn; it condenses in the muffler. Its only purpose to to provide lubrication to moving parts, such as needle roller bearings, lay down a thin film for between the cylinder and cast iron piston rings, and a few more places. A new engine need extra lubrication in life until the moving parts "seat" against non-moving parts. Without extra lubrication, you risk early, permanent engine damage, such as scored cylinder walls. Some riders recommend 16:1 oil ratios. That will not hurt the engine, either. They are just spraying for mosquitoes.
On one bike I have, I remain at 24:1, even though it has only 125 miles on it. Goo still drips out of it. I consider that normal.
On my bike, I switched to full synthetic Amsoil Saber (only because I could easily find it) about 220 miles ago, but after break-in. I have been running my bike pretty hard at times at 50:1. There is no goo.
Sparkplug air leakage... In your case, maybe oil in the threads and a little bit more torque on the plug provided a better seal. Air leakage should not occur, but it did on my bike as well. I used a light coating of thread sealer (NOT thread locker; it is very different) to seal the base of the plug against the cylinder head. I think I got a little tube of sealer from Auto Zone. If you want to try it: Place a light bead on the upper two or three threads farthest from the firing tip. If the sealer oozes a bit from the plug seat upon tighting, that is good.
Black plug tip... That results from the factory-provided main jet in the carburator. Its orifice size is two or three sizes too big. I recommend you research a few threads for additional information before doing anything to the main jet. The too-big main jet is allowing excessive fuel into the cylinder. Not all the fuel burns completely, and little amounts of carbon result. It either goes out the pipe and mixes with unburned oil to produce that black goo, or the carbon deposits itself on surfaces, such as your cylinder head, piston head, spark plug, etc. I have the same thing; it does not bother me. I clean the spark plug once in a while with a wire brush, but there is no noticeable increase in performance. Lately, I have been ignoring it. I will pull the plug for inspection if my bike engine will not start.
I strongly encourage wearing good head gear! I wear a full motorcycle helmet on every ride. Have fun riding!
Mike J