What I found most disturbing in Knox's comment about the "most beautiful black man he ever met", is not his looking but the fact that she never revealed his name.
Which would indicate that:
a: she can't remember it or have a hope of saying it right
b: she didn't know it
c: it was Rudy Guede and she was protecting him from a police investigation about a murder that hadn't even occurred yet.
I find it disturbing because you may count black men attending bars in Via Garibaldi just like you know your neighbours. Can you imagine how small the place is? How little the bars? Who is this unknown second black man?
Perugia has 160,000 people in it. About two thousand are 'black' as an American would understand it. Via Garibaldi is in an area notorious as a hangout for North Africans.
Perugia is famed for it's University (just around the corner) that takes on forgeign students on purpose - even from Africa, especially North Africa.
Nothing reported about Perugia would make it surprising to find multiple 'black men' on any given stay, in any given area.
It should be extremly easy to identify him and find him if we just had his name.
And thus a solid reason for Knox to never reveal his name.