[92snowmachine]

hopefully your dark coolant and the leak are unrelated. i have seen the small hoses that go to the oil coolers leak, sometimes only when under pressure. it creates a clean spot because the coolant cleans gunk off your engine and makes it oily. it would be unusual to have an internal and external head gasket leak if you had no previous signs of trouble. overflow reservoirs don't need to overflow if the coolant has another place to go, or you are not that hot.

you can test for a head gasket internal leak with this for $30 or less.
an external leak you will just have to run it and watch for leaks.
how does the fluid in the radiator look?