Maechen is the person in the black mage sphere. I was playing with my cousin once, and we were confused as well, and we reaserched both X and X-2 for info on him. Here is what we found.

In X, on the Mihen highroad, Maechen explains that he travels around Spira, learning of it's history. He then confesses that he forgets most of it, and can't even remember how old he is. This is very important.

Next is the image of Zanarkand that Seymour shows the party. This is an image of the real Zanarkand, and we used it for comparative visual purposes.

On to the evidence in X-2

First is the actual sphere. The speaker (Maechen) exclaims "how wonderful it was to shake her hand like that." Maechen being the forgetful person he is, later forgets everything from the sphere.

Later, when Maechen shakes Yuna's hand during chapter 1, it sparks this old, forgotten memory of his encounter with Lenne.

Later still, during chapter three, Maechen finds Yuna in Leblanc's chateu, having finally connected his forgotten memories. However, he loses the connection right before saying it.

Finally, the sight of Zanarkand and the connections that he had begun to make allow Maechen to realise that he is unsent, and he showa Yuna before sending himself.

This is how myself and my cousin pieced everything together. , Maechen was a black mage in the city of Zanarkand. Before the war started, he saw a concert of Lenne, and shook hands with her after. He recorded the s handshake sparked the memory of the handshake.

Next is how the black mage sphere got onto gagazet's peak. When Yu-Yevon realized that Zanarkand was going to be destroyed, he commanded everyone in the city to go to the summit of Gagazet, where Fayth scar was formed to create DZ. But Maechen was not sealed into the scar, so he left the recording he made and begun to travel Spira. Over time, he died, but he forgot both the fact that he was dead, and every spell he knew as a blackmage.

That is pretty much what we pieced together. The only plothole that we see is why Maechen in particular was not sealed in fayth scar with the other people of Zanarkand.

Also, the city in the black mage sphere lookes line DZ is because the city in the sphere is Zanarkand, and DZ is the memories of the people in Fayth scar of the real Zanarkand.