I like the concept, but if it is to be as rare as you say I feel like it has to have another gimmick entirely, maybe something that removes it from the RNG completely by having it modify existing equipment and engrams or having it decrypt into any equipment in the game. Maybe having it modify existing would be the more interesting choice by [u]assigning black engrams exotic or unique situational attributes similar to those found in the vault drops[/u] that can be added to anything non-exotic creating a whole new class of item - [b]Black colored Uniques[/b].