Maybe, but an off-hand mention of a character having sickle-cell is much more likely to imply the character has African or Mediterranean ancestry, since there was nothing to imply it was an unusual case. Taken together with the guard's racist remarks at the beginning (I don't know why people are looking at these separately, expect that it lets them say 'but not all--!' more easily), it's a PRETTY DAMN BIG CLUE that Shadow isn't White.