Our yucca is constantly developing black tips on its spines. I have trimmed them, but eventually the black ends return.
What to Do With Black Ends on Yucca Spines
Certified GKH Gardening Expert
This may be a reaction to chemicals in the water, if the plant is being watered from your tap instead of from rainwater. Try setting a pitcher of water out the night before to allow the chemicals to evaporate and then use the water on the plants.