The term doesn't have to be derogatory nor does it necessarily refer to cheap, or poor quality liquor. Any liquor or beer popularly mentioned in rap lyrics can be considered Blackahol.
(white guy #2) "sucka you know I ain't picky, just get me some blackahol and I'm straight"
(white guy #1) "werd"
"what are you drinking?"
"man, i'm sippin on some blackahol up in heah!"