Undergrowth — Search your library for a black card with mana value less than or equal to the number of creature cards in your graveyard, reveal it, put it into your hand, then shuffle.
A land card that produces black mana, even a Swamp, normally has no color.
Because tokens aren’t cards, they never count for undergrowth abilities.
Creature cards with other types, such as artifact creature cards, count for undergrowth abilities.
If a card in your library has {X} in its mana cost, X is considered to be 0.