Sins darken your heart

No! Rather, the stain has covered their hearts of that which they were earning.

[Qur'aan 83:14]

The Messenger of Allaah (sallallahu alayi wa sallam) said:

Verily, when the slave (of Allaah) commits a sin, a black spot appears on his heart. When he refrains from it, seeks forgiveness, and repents, his heart is polished clean. But if he returns, it increases until it covers his entire heart.

[Jaami al-Tirmidhi]