Fresh leaks wouldn't be black, and should look wet.
Which hydraulic systems are installed in the left BLG bay ?
Probably all of them.
There are a number of things in that left body gear wheel well, including....
Servicing point for all 4 hydraulic systems.
Gear (#1)
Body Gear Steering (#1)
Inboard flap drive motor (#1 Hydraulics)
Brakes and antiskid (possibly 3 hydraulic systems, depending on the location of shuttle valves).
etc...
Stains can also be caused by grease, oil and rainwater (plus contaminants, such as brake dust).