Bley is a fan term used to describe “bluish grey,” referring to the updated grey colour introduced in LEGO sets in 2003.
This colour was part of a broader effort by LEGO to standardize and improve the colour palette of their bricks, resulting in a more visually appealing and cohesive look across various sets.
The term “bley” has since become commonly used within the LEGO community to differentiate between the older grey colours and the newer bluish-grey variants.