If you still see standard light-gray inventory windows, the normal start background, or classic stone buttons after selecting Black Keys Resource Pack, a stalled reload or another interface pack is usually overriding the dark textures.
Force-Reloading Textures In-Game
1. Enter a world where keyboard shortcuts are active.
2. Hold F3 and press T to force Minecraft to reload all active resource packs and menu textures without closing the game.
3.
Removing Conflicting Menu Overrides
1. Open Options > Resource Packs and look at the Selected column on the right.
2. Check whether any custom UI pack, accessibility contrast pack, or full texture overhaul is positioned above Black Keys Resource Pack.
3. Click the up arrow on Black Keys to move it to the top slot, or move competing UI packs back to the Available column on the left.
4. Click Done and return to the start screen to confirm the solid black buttons and dark background are active.