Why is black oil in my coolant?

Hello, it wounds to me that you could indeed have an internal engine oil / coolant breach -- there really is no other way for oil to get inside your cooling system other than pouring it in on accident. For sure, don't jump to any conclusions -- have a professional diagnostic test performed that can determine that there is hydrocarbons actively getting into your cooling system, which would confirm something internal to the engine failing - such as a cylinder head gasket leak, cracked cylinder head, or similar. Good luck.

IF automatic transmission - possible fluid cooler *inside the radiator* has ruptured - difficult to identify lubricant type after it has mixed with antifreeze.