I've seen plenty of "Blueberry" Oscars over the years, but I've never seen one quite like this. But people are doing some nasty things to fish these days trying to increases sales. I would definately inquire as to why it is different before purchasing it, and don't always take the seller's word. If they are cruel enough to torture an animal for profit they are bad enough to lie for profit.
Also, FYI - The vast majority of dyed Oscars die within a month of being purchased, even when kept in proper conditions. The Blue also wears off rather quickly. I have one that I babied through it's first year and is now 5+ years old. He has several dark spots on him that look like bruises, he's not a pretty fish.
If it is natural, as mentioned, he will outgrow this unique feature quickly. Oscars are well known to have significant pattern changes from baby to juvi stages.