I agree with Michiganteddybear.
This past summer I had the same issue start with my bike. It looked exactly like your pics. After longer rides, I started noticing black/grey drippings on my rear wheel rim while parked. It was very thick and not really wet at all. I was very troubled for a time. I was sure, as you seem, that some seal inside had worn out, causing the gear oil to leak.
When it recently came time for a new rear tire, I cleaned the wheel, splines, pumkin/hub, etc. very good. I had a new tire installed on the wheel, and when I was putting it all back together, I painted moly 60 on all the proper places. I say "painted", to describe a thin coating placed with a small brush (picture painting size), not globs placed with a spoon or ice cream stick type of tool. This was the manner described on the moly 60 package.
I think even if there wasn't too much grease on the wheel splines before, that it broke down over time and became thin enough to sling off when things heated up.
Since my new tire has been on, there have been no more drops of dark mystery sludge on my rim, even after longer rides.
By the way, while doing this I did check & lube the driveshaft splines & universal joint. They were DRY, but not yet worn. There was a dark powder left, but NO grease. I think I got it just in time.
Best of luck!