Blight-Breath Catoblepas

The value of X is determined only as the triggered ability resolves. Once that happens, the value of X won’t change later in the turn even if your devotion to black changes.

If an activated ability or triggered ability has an effect that depends on your devotion to a color, you count the number of mana symbols of that color among the mana costs of permanents you control as the ability resolves. The permanent with that ability will be counted if it’s still on the battlefield at that time.