Ferns are outside where I have had excellent luck in the past. One has continued to have its fronds turn black and is covered now with bare stems. The one next to it is starting to do the same thing on the side facing the ill plant.
Boston Fern Fronds Turning Black
Certified GKH Gardening Expert
Your fern may have some environmental issues cause this.
To much light, heat, watering issues--to much or to little.
Wind can be harmful to your fern also.
A cool location with some filtered light, away from wind. Water daily and even twice a day if the temperatures are above 85 degrees.
You can clean up the dead and damaged leaves.
Watch for insects or disease as a stressed plant can invite these things.
Here is a link that has more information.