i think its deffenately mold. iv been growing for a few years and frow what experience iv had is its mold. this can occur from needle inocculation or un sterile jars in your case its probobly from having unsterile tools while seperating the origional brf cake for curther colonization. keeping them on the other hand is possible when the new cakes are fully collonized u can cut out black sections and break the cake up with steralized gloves into a small mixture of coir verm, and pearlite this will increase your yeild and protect your mycelium from further airborn containants. tho any mushrooms which grow from the black will be more slimy, dark, and fragile. this fruit might make u sick so i would toss any that occur. if your jar feels wet and mushy or the black covers the cake i would advise u toss this. this may damage any other cakes your fruiting with. i hope it helps feel free to ask any questions.