G'day wine man,
from the photos it doesn't look like you've hit anything. Can't imagine it's a metal fatigue issue unless that one bolt has been massively overtightened and it doesn't seem to be as there's no apparent distortion. That's just a bit of a guess though. Looks rather like a manufacturing fault.
Use a steel screwdriver and drag it REAL hard over the case. Aluminium will get scraped off much more than steel so you should get an idea what it's made of pretty easily.
When you change oil there's always a bit left in the case so the new oil may well be a little discoloured. I don't know why trans case oil should be really dark though. Never had that on two models. Shouldn't be running at a temperature high enough to cause carbonising/blacking unless there's a real major problem surely.
Reckon I'd run it a while and keep checking the oil colour. If it blackens you may be up for a take out and inspection but I'm not experienced in that.
If the case comes off, get the crack fixed otherwise keep a real sharp eye on it. Third man's suggestion may well be worth a try in the meantime.
Luck,
Col