I don't think it's AT... When i see this coin, i remember the one lincoln "fire and ice" coin a member owned on here.
Well if its a proof, my vote is it could be natural. 19th century copper proofs CAN tone this color. There are written accounts of blue proofs being sold in the 50's. Yes, MS70 is responsible for most blue lincolns, but my vote is that this is NT of a proof.
Silver proofs can tone that colour also, I own an 1880 half that I bought from our own LostDutchman, aka Matt, more than a few years ago that is in a PCGS holder and is a rosy blue hue. It bucks the norm for me, usually I like my ladies blast white without blush etc - but this one was just so lovely I couldn't live without her.
Thing abouts proofs, though, is in hand they can look completely different than photos. I mean way more than business strikes especially concerning toning. I would NEVER buy a toned proof with pretty color from a photo unless i had an absolute right to return it at no cost to me. I have seen just absolutely gorgeous proofs in photos look blah in hand.