As with all the Chieftains of the Dúnedain, Aravorn's date of birth appears only in The Peoples of Middle-earth (Chapter VII The Heirs of Elendil). It cannot therefore be considered completely reliable.
2
The translation 'Black Lord' relies on breaking down the name as ara-vorn. It could also be interpreted as ar-avorn, which would mean something like 'steadfast lord', though this derivation is somewhat less certain.