I'd get the Kawasaki service manual. You can get it from your dealer or any online parts supplier.
The black oil is what we call "Spooge" or "Splooge", depending who you talk to. It is a result of an bike that is jetted to rich, i.e it is getting more fuel through the carburetor than it can burn in the combustion cycle.
While it is normal for a new KDX to have a spooge problem, it is not something that you want your bike to do. Overtime, it can lead to some expensive problems.
To fix the problem you will have to rejet the carburetor on the bike. If you are new to dirt bikes, this can seem a little intimidating. Some dealers don’t like to do this work so it is something that you should learn to do. It’s worth the effort to learn because it is something that you will want to have done to the bike as you gain experience and/or modify the bike.
You’ll find a lot of useful information in the FAQ that is at the top of this forum. Welcome to the wonderful world of KDX’s (and DRN too!).