Coud you please post a photo of the fish? A lot of goldfish lose their black color eventually due to melanophore migration. Where there is no ultraviolet spectrum coming from the direct sunlight which is responsible for maintaining the black color, the melanin cells are not replaced quickly by new ones hence the black subsides and replaced by other colors instead. I have an oranda, Velvet whose black color is changing to orange with tinge of olive green and chocolate. My black ranchu, Loco is also changing his colors as he is showing orange shades near his belly already. I am still unsure if my 7" black oranda will ever change his colors yet. You cannot retain the black color unless the fish has exposure to the sunlight which is responsible for retaining the melanin cells.
There is one other thing to note. The black smudges that may appear on your goldfish is not color development but indication of ammonia burns. Only way to correct it is doing plenty of water changes. This is also associated with red streaking on the fins.
By 8 months, the black moor should already be 6-7 inches but growth rate is also influenced by dietary requirements and genetics aside from environmental conditions. TwistedPenguin is correct. That black moor needs a large tank if your girlfriend hopes to keep him for a long time. A 30g can fit two adult fancy goldfish just fine but a 55g would be a much better option and fits at least 3 fancy goldies in it. A 75g has better floorspace area. Craiglist has deals most of the time.