Hi Claus,
What you were thinking is how I would interpret the rules.
1) If a young'un has black orc blood then at any point on or after they reach 25 xp they can gain the "Proven Warrior" skill as a skill after an advance that essentially turns them into another black orc with the same benefits. Before 25 xp though young'uns with black orc blood are simply expensive young'uns.
2) You have to spend 10 gc when you hire a young'un with black orc blood. This represents the rarity of black orcs. I guess this also acts as a gamble that the young'un may die before getting any benefit from its black orc blood but on the flip side if it survives then it will eventually become more lethal than regular orc heros.
I hope that this helps.