Hey BK , First off i'd definatley run a pH test dude

make sure thats about spot on , i can make out rusty type looking markings which do indicate pH fluxuations.. erm.. but black spots ? .. the only thing i know of with little black spots can sometimes actually be insect shit !.. maybe you do have some kind of little something in the growroom ... if however it is something along those lines i really do not know what to suggest to use , some kind of spray .. im not too sure ive never encountered a bug problem.. please can anyone else help shed some light on this situation of BK's... bk !! wish i could be of more help my friend !