The Black Hammer sniper rifle, like all Crota's End weapons, initially came with an upgrade called Hive Disruptor. The activation of Hive Disruptor on this weapon made the weapon's primary perk White Nail work improperly, so Bungie disabled it. Until Bungie re-enables it, your Black Hammer technically isn't fully upgraded. That's why your weapon does not have the yellow lining around the icon.