Is 白加黑 ("white add black") slang for 感冒药 ("cold medicine")?
白加黑 

It is not so much a slang but a general term for cold medicine. When a brand is so popular, people would use it to describe the product itself, like 公仔麵 (instant noodle)  and 花奶 (evaporated milk) in Hong Kong

白加黑:



公仔麵:



花奶