Thanks for the nice photos. They really help.
I don't know what the black item is, but it does look like it has a return. However, if it is a return, it seems too small for your intended use.
Can I suggest thinking about something like this? If you were to use a piece of thin wall tubing with an OD that matches the ID of your hydraulic flare to thread return fitting. Imagine that it has a length just longer than the inner length of the hydraulic fitting. If the end of the tubing were then bent 90 into an "L" shape, the long portion would slide into and would fit inside the fitting. You could unscrew the fitting, place the 90 degree tubing in, then insert the tube into the threaded hole and angle the tube downward as you insert and rethread the hydraulic fitting in. Because the tube can swivel, you can thread the fitting in, and the end will point downwards, where it will direct the bulk of the return flow. If the horizontal portion in the fitting is longer than the diameter of the fill pipe, then it should not fall out.
Does that make sense?
Another alternative is to take the return hose off. Take the tank fitting/ adapter to a local welder and have them weld a small plate or tube directing the flow downwards on the end of the fitting or across the pipe.
Both of these will slow adding replacement hydraulic oil to the tank, but hopefully that is not a common occurrence for you.
All the best,
Peter