Armor slots in inventory are black in the following cases:
This list might be incomplete.
Armour slots Turn Black when banners are held
Banners in hotbar affect armor and offhand slots
Armor slots black when hotbar full
Armor slots of creative inventory will display black when not held items and player having invisibility state.
Chest or possibly other non full blocks cause graphical glitch.
when chest/shulker box is in your hotbar it causes glitch
Dark Armor Slots
Armorslots go black if hotbar is filt with banners
Creative menu corners have opacity issues; outlines have no transparency
Offhand slot is glitched / black in some circumstances