All passive mobs and aggressive mobs have blacked-out textures with the odd exception of the player and pigmen/zombie pigmen. Item frames are blacked out but maps placed in them are normal, signs are blacked out but dyed signs become readable. All blocks and most items are blacked out in inventory. All menu text except the logo disappears until you load up a world and back out, all text in-game is untouched. All of this happened when I updated to 1.17, which returns to normal if I go back a version. Also, the black void behind the invisible player model is a painting, all paintings are blacked out as well. Other than that the game runs just fine, it's just mildly terrifying to turn a corner to an exploding black blob.