I can't decrypt my Engrams, they will not appear on Master Rahool's decrypt list.
Update Bungie said I have to wait till the 19th to use them, it appears even the blue Engrams as well.
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It is because the blue (or purple) engrams that you have obtained contain HoW items. You get these from the fallen caches after doing a wanted bounty. He will still decrypt engrams you have picked up off the ground.