Higher no3 does not likely have any effect on darker corals on it's own, but it is likely a sign that more nitrogen passed through the system. Those other sources of higher nitrogen could have produced more
zoox in the corals making them appear more dark to our eyes. Some like this darker color and some do not. If you rise too much, the coral will not be able to protect the
zoox from too much nitrogen anymore and it can start to expel
zoox. Nitrogen is necessary for new tissue, can be used well in a slight surplus, can be a hinderance in too much of a surplus and then it will end in death of all organic tissue if it gets too high.
With corals, this all can happen from undetecteable to 5ppm for some that are very sensitive whereas some corals still go strong in the undetectable to 150ppm range, or more. What makes this even harder is that too few know if a growing coral can be growing faster (in the hinderance range, but still alive and barely growing).
The age-old example is ORA red planet. In higher no3 and po4 environments, it would get a deep, dark red color - excess
zoox. In lower no3 and po4 environments, it would have white on the tips, grow faster and have white contrast in the middle of the branches and stuff. Some folks like one look, some like another and some like either of them.
Remember, though, that it was not likely the rise of no3 that did this. It was the availability of nitrogen in different forms that got more nitrogen to the coral and also made the nitrate on the backend rise.