When standing in the middle of soulsand block*, any item (or just a hand) held by the player in first person view is viewed at light level=0, results in very dark textures of both items, and the hand. This was tested with items such as almost all diamond tools held in hands, nether wart, enderpearls, and glowstone blocks. Besides darkness of items increasing, the sky becomes much darker even though the player is surrounded by sources of light.
To reproduce:
1. Place soulsand
2. Grab any items and use standard first person view
3. Scroll through the items in the hotbar - you will see that they have very dark textures.
4. The sky will darken as well even if it's daytime

*Note: If you stand near the edge of soulsand block, textures won't darken. It happens only when you stand in the middle of the soulsand block.