Shulker boxes are notacibly darker than other blocks and entities. This is related to the lightmap (so NOT the textures) and using mods to change the lightmap can make this obvious (but to be clear this bug applies to unmodded MC).

They appear to be always one light level below their surroundings (I pinpointed this using customised lightmaps but to reiterate: without modifications or anything it acts just the same).