If a card in a player's library has in its mana cost, X is 0 when determining that card's mana value.

Hybrid mana symbols, monocolored hybrid mana symbols, and Phyrexian mana symbols do count toward your devotion to their color(s).

If you put an Aura on an opponent's permanent, you still control the Aura, and mana symbols in its mana cost count towards your devotion. Similarly, if you make an opponent the protector of a Siege you control, mana symbols in that battle's mana cost count toward your devotion.

Colorless and generic mana symbols (, , , , , and so on) in mana costs of permanents you control don't count toward your devotion to any color.

Count the number of black mana symbols among the mana costs of permanents you control as Grim Servant's triggered ability resolves to determine your devotion to black. If Grim Servant is still on the battlefield at that time, it will be included in that count.

Mana symbols in the text boxes of permanents you control don't count toward your devotion to any color.