It should be noted that the majority of the card stock boarding passes (both gold and blue), still use a thermal printing mechanism. Try it out -- hold a card stock BP up to a lightbulb or other heat source and watch it become black all over. Also this is why even the card stock BPs fade over time.
The only exception I've found was in MEL where the card stock BPs were printed with actual ink. This explains why my MEL BP is still nice and black three years later, while all other BPs of the same age have long since begun to fade.