I have experienced my ferns turning black in several occations, when i moved them from a low light, low CO2 tank - to a higher lighted, high CO2 tank.
I dont know whether its the adjustment to better/differnt conditions, that makes the fern to "shut down" the old leafes (these start to form new rhizomes, as a propagation method), but soon new and fresher leafes starts to form form the old rhizome.
I just trim the big, old black leafes away, and soon you have e nice fresh plant...
Old plant
Same plant a few weeks after, and after i trimmed the old leafes away:
