The heart develops black spots due to sins

the Messenger of Allah said: “Verily, when the slave (of Allah) 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. And that is the ‘Ran’ which Allah mentioned: ‘Nay, but on their hearts is the Ran which they used to earn.’”  

عَنْ رَسُولِ اللَّهِ ﷺ قَالَ إِنَّ الْعَبْدَ إِذَا أَخْطَأَ خَطِيئَةً نُكِتَتْ فِي قَلْبِهِ نُكْتَةٌ سَوْدَاءُ فَإِذَا هُوَ نَزَعَ وَاسْتَغْفَرَ وَتَابَ سُقِلَ قَلْبُهُ وَإِنْ عَادَ زِيدَ فِيهَا حَتَّى تَعْلُوَ قَلْبَهُ وَهُوَ الرَّانُ الَّذِي ذَكَرَ اللَّهُ {كلاَّ بَلْ رَانَ عَلَى قُلُوبِهِمْ مَا كَانُوا يَكْسِبُونَ} [المطففين 14]  

The Day when mankind will stand before the Lord of all tat exists He said: “They will be standing in sweat up to the middle of their ear.” – Hammas said: To us it is Marfu - .  

حَمَّادٌ هُوَ عِنْدَنَا مَرْفُوعٌ (يوْمَ يَقُومُ النَّاسُ لِرَبِّ الْعَالَمِينَ ) قَالَ يَقُومُونَ فِي الرَّشْحِ إِلَى أَنْصَافِ آذَانِهِمْ  

‘They Day when mankind will stand before the Lord of all that exists.’ He said: “One of them will be standing in sweat up to the middle of his ears.”  

عَنِ النَّبِيِّ ﷺْ ( يومَ يَقُومُ النَّاسُ لِرَبِّ الْعَالَمِينَ ) قَالَ يَقُومُ أَحَدُهُمْ فِي الرَّشْحِ إِلَى أَنْصَافِ أُذُنَيْهِ قَالَ هَذَا حَدِيثٌ حَسَنٌ صَحِيحٌ وَفِيهِ عَنْ أَبِي هُرَيْرَةَ