I got the black hammer and spent time and resources upgrading it. It is fully upgraded, but there is no yellow highlight around it, and it doesn't cause damage to enemies. I can't find an email to contact bungie/destiny anywhere.
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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.