Why on Earth we were reading 4 books and only in the 5th the true heir appeared? It just seem strange to me! It is not an evidence that Aegon is a fake, I just wonder why? All that we were reading before would make no sense, because we haven't even know this guy to like him (I always thought that one of the main character should be a king rather than some stranger).
Don't get me wrong, I do not know whether he is fake or not (although, I prone to think he is a false dragon) and I think his appearance makes a wonderful twist, but it seems a little bit (I don't know even how to call this…well let it be) strange to heir and a PtwP to be discovered almost at the end of a series. it is not logical. Unless GRRM wants to impress us, but frankly I would be disappointed, if is a real heir and will sit on the Iron Throne. I hope that Varys and Illirio fail their purpose, I don't like what they do.
I do think that he is a Blackfyre, but neither he nor his companions know that he is not a Targ. The boy was raised to thinking that he is.