I've had this darkening happen a lot with a certain widely available brand of e-liquid,but with a 70PG/30VG mix ratio. Sometimes it tastes burnt,but on one occasion although the liquid was almost black it was an incredibly smooth vape to the point that I was disappointed when the tank was empty :-) I've just had an irritating run of three poorly factory-built Vision attie heads. They tasted burnt from the word go. Despite flushing them through,the reason was clear.....all three coils were wrapped far too tightly around the wick,so I've just rebuilt them and they taste ok now. They all measured 2.45 ohms but a good reading certainly doesn't mean they're not going to concentrate the heat on one spot of the wick and strangle it.