Hi,
I want to give my magus to 'Manius the Black' or 'Manius Black' in Latin.
Manius Nigra is ok?
Manius Nigerius should be correct... Though my grammar is rather rusty, so I might be off. 
It really depends on what sense you intend by "The Black". If it is a reference to his deeds or some inherent evil nature/disposition then you have a much wider range of options than if you simply mean "black skinned" or the colour black (which wouldnt make much sense as a cognomen within Latin conventions anyways I should think).
Some excellent resources on how to develop a Roman cognomen (nickname) that I found are:
or an active society you could email to ask such questions to:
Thanks!
He has black skin. So Manius Nigerius sounds good.
You also could call him
Manius Attelus (meaning "Dark Skinned") which might ring better to modern ears anyways.
Taken from this link:
novaroma.org/nr/Choosing_a_Roman_name
(acroll down to the list of names with meanings.
Which leads me to ask the universal question: What kind of music do black-skinned magi prefer? Reggo perhaps? 
I don't care American political issues. America wasn't discovered in the time of ArM. 
But I found there the cognomen Niger! Thanks for the link! 8)
Exactly. 
So then, you need to start the first ARM5 band "Manius Black and the Dreaded Locks!
alternatively you could give him a name based on where he's from.
Manius the Ethiope, (whatever that'd be in latin), etc.
He's black because of his fairie heritage.
Thanks anyway the idea.