hmmm....... you might check those heater hoses, where they go into the firewall, for leaks. A leak from them could be following the firewall down and dripping onto the top of the transmission.. ??
I suggest you reach back and get a sample of the dark stuff and see if it feels and smells like oil.
Maybe clean off the top of the transmission with something like brake cleaner and see if the "stain" returns.
OR, wait until someone who knows what they are talking about, to come along and chime in ......
humfrz