how is the rest of the plants in your tank ? Nothing else showing these signs?
Is this a relatively new plant. Could be a molt spurred by the change in venue.
Too much light possibly or not enough ferts cold be an issue... I had a java fern in a high light tank with plenty of other plants. The Java fern was shaded some but not enough. All the leaves started turning black. This could also been the result of being a new plant and molting some and some early fertilizing/CO2
issues I had. But too much light means the plant might be trying to out grow the rest of its resources which means its looking for food therefore a deficiency. Java fern do well in very low light situations. SO its possible that maybe this one is receiving more than it needs.
Using the chart below nitrogen phosphates potassium deficiencies all have burned edges moving inwards and consuming the leaf.. So if you were deficient in all of these do to too much light I could see the extreme burn being a possible sign. As I mentioned I had some fert issues early in my tank and I was woefully under fertilizing as well as my light was way to close. That combo produced real burnt black leaves. I have since replaced it with high light stem plants
Food for thought