Exactly what it sounds like. The red-purple sputum that is a mixture of cranberry
pulp and the excess mucus caused by irritation from drinking the acidic
juice. Similar substances can come out of drinking lemon or grapefruit
juice, but those tend to be a less alarming and therefore noteworthy color.
"Holy
shit, did you just cough up blood?!"
"No, that's just cranberry flem; I drank some cranberry
juice earlier"
"Oh...
Gross"