I assume you washed out the transmission with kerosene? Yes, that could have contributed to it, especially if what your seeing is transmission oil instead of engine oil. It may be black because it's carrying out clutch dust as it runs down.
I think I'd go ahead and run it awhile and see what happens. Mine ran for years with the front trans seal leaking, finally got time to fix it, didn't seem to hurt anything.
I assume your oil levels are good, not overfull. Might want to watch both and see if you develop a pattern of consumption, then keep the level just below full, see if that helps with the leak until you can get in there and fix them... Them as in do both while it's apart.