Well, yes. If you have a deck with more than one colour, then you would also have to look at the amount of coloured mana in the costs, in order do decide how many of each type of land to play. In this case, however, all the cards in the deck are black, and all the lands produce black mana. That means it doesn't matter if a cards costs B2 or BBB, as it will all be paid with black mana anyway.