We’ve updated Creator Rules (1.15.1) to allow unrealistic blood-like substances (e.g. not red in color) to be used in your islands with fantasy NPC creatures. These effects cannot appear in thumbnails, or be used with any Fortnite Outfits or human-like characters.
Realistic violence and gore of any kind is still not permitted.
See the Creator Rules here: https://www.fortnite.com/news/fortnite-island-creator-rules