ennegrecer
blacken
en-
to make or cause to be
+
negro
black
+
-ecer
verb ending indicating a process
Ennegrecer means to blacken, which can be understood as en-0 (to make or cause to be0) + negro1 (black1) + -ecer2 (verb ending indicating a process2).
verb
- blackenTo blacken or darkenEl humo puede ennegrecer las paredes. // Smoke can blacken the walls.