I have a cell line that causes DMEM to turn a dark purple/black colour when the cells are 100% confluent. However, this only happens when the cells are stably transfected with a specific plasmid, the media does not turn black with the original cell stock, nor when transfected with other plasmids. The media is not turbid and all cells are kept in the same conditions.
DMEM normally contains phenol red indicator, which can turn purple in alkaline condition.
It is possible that your plasmid is the one which provokes the pH change. High chlorine (or other proteins associated with your plasmid) can cause this.
It is also possible to turn purple if your media was left outside of the 5-10% CO2 incubator for a very long duration of time that it affected the bicarbonate buffer system.
Black is quite special...maybe it turns from ret to yellow, but black is untypical. You should contact the producer/supplier of your media.
Purple or black? If it is purple, the medium has turned alkaline and should be changed, perhaps for a medium with added HEPES (20-50 mM), to increase buffering capacity. Is your incubator working correctly, is the CO2 supply on? If you take a sample of the medium and add some dilute acid, the phenol red indicator should turn yellow.
I have never seen black, though. What kind of plasmid is it, does it encode for an enzyme whose product is black?
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