From my understanding, there is no way CJD can be transferred from something such as an open wound. It'd have to come in contact with your brain or the vitreous fluid in the eyes (also only possible via surgery) hence why those incidents of contamination only occurred in people who had brain surgery. Also, the statistical probability that the person who left the blood even had CJD is ridiculously low, like nearly impossible low. People who have CJD would not be wandering around a store, bleeding either due to the debilitating nature of the disease. Also, the chance that the black stuff was something biologic in nature is also pretty low. It could have been adhesive, old sticky food (having a four year old, I know that enough sticky stuff can get hair and dirt in and turn into black gunk that looks pretty scary), etc.
I know this stuff is scary and I know how hard it is to stop obsessing about it so it's not a problem answering :)