You need to remove the lower unit and have it psi tested...it will fail...but will tell you where the leak is. Will cost you a couple hunderd to have the seals all redone. Well worth it as water in the lower is BAD...as you already know. Milky can happen with only a few drops of water. If you are handy you can do the seals yourself with the service manual. It will show you alot of tools that you "have" to have..but like I said...if your handy you can do it....without the special tools in most cases.
When they psi test they will put 15 psi or so on it with a hand pump..normally screwing fitting into the drain plug. They will then see if it will hold psi for 30 mins to an hour...well of course yours has milky oil....SO...it is leaking...so they will probably just dunk it and see where the bubbles come out...this will show them the bad seal. It is MUCH easier for water to get in than for oil to get out...if you get oil leaking out you'll get ALOT of water in it. They will also pull a vacuum on it. Before and after the sealing job.
The Black you are talking about is carbon from gas/oil mix setting up in cylinders. Do a decarb soon. You will find instructions to this under FAQ at the top of this forum. That is why its black...seriously doubt it is from LU.
Have the LU resealed...or do it yourself...and do a good decarb...and GO FISHING!! Good luck...let us know if you have any other questiosn.
"But if serving the Lord seems undesirable to you, then choose for yourselves this day whom you will serve, whether the gods your ancestors served beyond the Euphrates, or the gods of the Amorites, in whose land you are living. But as for me and my household, we will serve the Lord.” Joshua 25:15