black stuff in arnold palmer spiked?

i was drinking an arnold palmer spiked out of a cup and when i got to the bottom i noticed some black stuff on the bottom of the cup. so i opened another one from the same box and poured it into a glass and saw it again. my mom had also bought some recently so i took one out of that box and poured it in a glass and there was no black stuff in the bottom. does anyone know what this could be??