The fact it has been going good for 10 years and just now starts to blacken is a bit strange. You'd think it might be cold damage but I understand you had much colder weather a few years back and obviously no problem then. Is there any chance that something was tipped onto the fronds that killed them? A change in the canopy/shade over the plant that let strong sun come through suddenly before the fronds could harden? If the ferns below on the ground are all okay then you'd think it couldn't be anything to do with the soil. Hope you can find the answer and rectify the problem soon. They're a beautiful plant (I assume you have Cyathea cooperi). I have a small one. My biggest problem is keeping the water up to it during dry and/or windy weather.

Hi
I grow mine outdoors also but in pots due to my poor soil.
I've also experienced that problem and was told it was a type of virus . They did recover in time but were distroyed by hurricane. You might try a general fungicide??
Be sure to get one safe for ferns !!! gary