I got this when I took HBC as well, and it seemed to vary from month to month in terms of how quickly it showed up. As asukasan says, my doc said it was the result of the blood having hung out and been exposed to air.

In the realm of further TMI...whenever my discharge has looked "odd" to me, I generally take a cautious sniff. If it smells normal (ie--like blood and/or kind of like meat), everything is okay. If it smells "off," I head to the doctor. Also, if my discharge had suddenly changed as you describe and if I had other symptoms like unusual pain or a fever, I'd head to the doc.