blackahol is any kind of liquor stereotypically consumed by urban ghetto type folks, often promoted by black rap artists. Examples would be the "fotey" (forty-ounce bottles of beer), malt liquor, thunderbird, and sometimes liquors like hennessy and couvosier.. or however that's spelled.
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 #1) "Nigga whatchu want on this beer run?"
(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!"